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diff --git a/old/67873-0.txt b/old/67873-0.txt deleted file mode 100644 index 39d17dd..0000000 --- a/old/67873-0.txt +++ /dev/null @@ -1,10894 +0,0 @@ -The Project Gutenberg eBook of Volcanoes: What They are and What They -Teach, by John Wesley Judd - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: Volcanoes: What They are and What They Teach - -Author: John Wesley Judd - -Release Date: April 18, 2022 [eBook #67873] - -Language: English - -Produced by: Tom Cosmas compiled from materials made availbe at The - Internet Archive and placed in the Public Domain. - -*** START OF THE PROJECT GUTENBERG EBOOK VOLCANOES: WHAT THEY ARE AND -WHAT THEY TEACH *** - - - - - - - -Transcriber Note - -Text emphasis denoted as _Italics_. - - - - -THE - -International Scientific Series - -VOL. XXXV. - -_Frontispiece._ - -[Illustration] - -Sections of Igneous Rocks, illustrating the passage from the glassy to -the crystalline structure. - - 1. Vitreous Rock. 2. Semi-Vitreous Rock. 3. Vitreous Rock with - Sphærulites. 4. Rock with Crypto-crystalline Base. 5. Rock with - Micro-crystalline Base. 6. Rock of Granite Structure built up - entirely of Crystals. - -[_See pp._ 63-68. - - - - - VOLCANOES - - WHAT THEY ARE and WHAT THEY TEACH - - - BY - - JOHN W. JUDD, F.R.S. - - PROFESSOR OF GEOLOGY IN THE ROYAL SCHOOL OF MINES - - - - _WITH 96 ILLUSTRATIONS_ - - - - SIXTH EDITION - - - - LONDON - - KEGAN PAUL, TRENCH, TRÜBNER & CO. Ltd. - - PATERNOSTER HOUSE, CHARING CROSS ROAD - - 1903 - - -(_The rights of translation and of reproduction are reserved._) - - - - -PREFACE. - - -In preparing this work, I have aimed at carrying out a design suggested -to me by the late Mr. Poulett Scrope, the accomplishment of which has -been unfortunately delayed, longer than I could have wished, by many -pressing duties. - -Mr. Scrope's well-known works, 'Volcanoes' and 'The Geology and Extinct -Volcanoes of Central France'--which passed through several editions -in this country, and have been translated into the principal European -languages--embody the results of much careful observation and acute -reasoning upon the questions which the author made the study of his -life. In the first of these works the phenomena of volcanic activity -are described, and its causes discussed; in the second it is shown that -much insight concerning these problems may be obtained by a study of -the ruined and denuded relics of the volcanoes of former geological -periods. The appearance of these works, in the years 1825 and 1827 -respectively, did much to prepare the minds of the earlier cultivators -of science for the reception of those doctrines of geological -uniformity and continuity, which were shortly afterwards so ably -advocated by Lyell in his 'Principles of Geology.' - -Since the date of the appearance of the last editions of Scrope's -works, inquiry and speculation concerning the nature and origin of -volcanoes have been alike active, and many of the problems which were -discussed by him, now present themselves under aspects entirely new and -different from those in which he was accustomed to regard them. No one -was ever more ready to welcome original views or to submit to having -long-cherished principles exposed to the ordeal of free criticism than -was Scrope; and few men retained to so advanced an age the power of -subjecting novel theories to the test of a rigorous comparison with -ascertained facts. - -But this eminent geologist was not content with the devotion of his -own time and energies to the advancement of his favourite science, for -as increasing age and growing infirmities rendered travel and personal -research impossible, he found a new source of pleasure in seeking -out the younger workers in those fields of inquiry which he had so -long and successfully cultivated, and in furthering their efforts by -his judicious advice and kindly aid. Among the chosen disciples of -this distinguished man, who will ever be regarded as one of the chief -pioneers of geological thought, I had the good fortune to be numbered, -and when he committed to me the task of preparing a popular exposition -of the present condition of our knowledge on volcanoes, I felt that I -had been greatly honoured. - -In order to keep the work within the prescribed limits, and to avoid -unnecessary repetitions, I have confined myself to the examination of -such selected examples of volcanoes as could be shown to be really -typical of all the various classes which exist upon the globe; and -I have endeavoured from the study of these to deduce those general -laws which appear to govern volcanic action. But it has, at the -same time, been my aim to approach the question from a somewhat new -standpoint, and to give an account of those investigations which have -in recent times thrown so much fresh light upon the whole problem. -In this way I have been led to dwell at some length upon subjects -which might not at first sight appear to be germane to the question -under discussion;--such as the characters of lavas revealed to us -by microscopic examination; the nature and movements of the liquids -enclosed in the crystals of igneous rocks; the relations of minerals -occurring in some volcanic products to those found in meteorites; the -nature and origin of the remarkable iron-masses found at Ovifak in -Greenland; and the indications which have been discovered of analogies -between the composition and dynamics of our earth and those of other -members of the family of worlds to which it belongs. While not evading -the discussion of theoretical questions, I have endeavoured to keep -such discussions in strict subordination to that presentation of the -results attained by observation and experiment, which constitutes the -principal object of the work. - -The woodcuts which illustrate the volume are in some cases prepared -from photographs, and I am indebted to Mr. Cooper for the skill with -which he has carried out my wishes concerning their reproduction. -Others among the engravings are copies of sketches which I made in -Italy, Hungary, Bohemia, and other volcanic districts. The whole of the -wood-blocks employed by Mr. Poulett Scrope in his work on Volcanoes -were placed at my disposal before his death, and such of them as -were useful for my purpose I have freely employed. To Captain S. P. -Oliver, R.A., I am obliged for a beautiful drawing made in the Island -of Bourbon, and to Mr. Norman Lockyer and his publishers, Messrs. -Macmillan & Co., for the use of several wood-blocks illustrating -sun-spots and solar prominences. - - J. W. J. - -London: _May 1881_. - - - - - CONTENTS. - - - CHAPTER I. - - PAGE - - INTRODUCTORY: NATURE OF THE ENQUIRY - - 1 - - CHAPTER II. - - THE NATURE OF VOLCANIC ACTION - - 7 - - CHAPTER III. - - THE PRODUCTS OF VOLCANIC ACTION - - 39 - - CHAPTER IV. - - THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS - - 67 - - CHAPTER V. - - THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS - - 112 - - CHAPTER VI. - - THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS - - 161 - - CHAPTER VII. - - THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOLCANIC CENTRES - - 186 - - CHAPTER VIII. - - THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE OF THE GLOBE - - 224 - - CHAPTER IX. - - VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY - - 247 - - CHAPTER X. - - THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE - - 281 - - CHAPTER XI. - - WHAT VOLCANOES TEACH US CONCERNING THE NATURE - OF THE EARTH'S INTERIOR - - 307 - - CHAPTER XII. - - THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN - THE CAUSES OF VOLCANIC ACTION - - 331 - - INDEX - - 371 - - - - -ILLUSTRATIONS. - -Sections of igneous rocks illustrating the passage from the -glassy to the crystalline structure - - _Frontispiece_ - - Fig. Page - - 1. Stromboli, viewed from the north-west, April 1874 _to face p._ 10 - - 2. Map of the Island of Stromboli 11 - - 3. Section through the Island of Stromboli from north-west to - south-east 13 - - 4. The crater of Stromboli as viewed from the side of the - Sciarra during an eruption on the morning of April 24, - 1874. 14 - - 5. Vesuvius in eruption, as seen from Naples, April 26, 1872. - (_From a photograph_) _to face p._ 24 - - 6. View of Vulcano, with Vulcanello in the foreground--taken - from the south end of the Island of Lipari 43 - - 7. Minute cavities, containing liquids, in the crystals of rocks. - (_After Zirkel_) _to face p._ 60 - - 8. Minute liquid-cavity in a crystal, with a moving bubble. - (_After Hartley_) 63 - - 9. Cavity in crystal, containing carbonic-acid gas at a - temperature of 86° F., and passing from the liquid to - the gaseous condition. (_After Hartley_) 64 - - 10. Monte Nuovo (440 ft high) on the shores of the Bay of Naples. - (_After Scrope_) 76 - - 11. Map of the district around Naples, showing Monte Nuovo and the - surrounding volcanoes of older date 78 - - 12. Outlines of the summit of Vesuvius during the eruption of - 1767. (_After Sir W. Hamilton_) _to face p._ 80 - - 13. Crater of Vesuvius formed during the eruption of 1822 - (_After Scrope_) 82 - - 14. Crater of Vesuvius in 1756, from a drawing made on the spot. - (_After Sir W. Hamilton_) 84 - - 15. The summit of Vesuvius in 1767, from an original drawing. - (_After Sir W, Hamilton_) 85 - - 16. Summit of Vesuvius in 1843 86 - - 17. Outlines of Vesuvius, showing its form at different periods - of its history 87 - - 18. Cascade of lava tumbling over a cliff in the Island of - Bourbon. (_After Capt. S. P. Oliver, R.A._) 93 - - 19. Lava-stream (obsidian) in the Island of Vulcano, showing - the imperfect liquidity of the mass 95 - - 20. Interior of a rhyolitic lava-stream in the Island of Lipari, - showing broad, sigmoidal folds, produced by the slow - movements of the mass 96 - - 21. Interior of a rhyolitic lava-stream in the Island of Lipari, - showing the complicated crumplings and puckerings, - produced by the slow movements of the mass 96 - - 22. Vesuvian lava-stream of 1858, exhibiting the peculiar - 'ropy' surfaces of slowly-moving currents. - (_From a photograph_) _to face p._ 98 - - 23. Vesuvian lava-stream of 1872, exhibiting the rough cindery - surfaces characteristic of rapidly flowing currents. - (_From a photograph_) _to face p._ 96 - - 24. Concentric folds on mass of cooled lava. (_After Heaphy_) 100 - - 25. Mass of cooled lava formed over a spiracle on the slopes - of Hawaii. (_After Dana_) 100 - - 26. Group of small cones thrown up on the Vesuvian lava-current - of 1855. (_After Schmidt_) 101 - - 27. Natural section of a lava-stream in the Island of Vulcano, - showing the compact central portion and the scoriaceous - upper and under surfaces 104 - - 28. Section of a lava-stream exposed on the side of the river - Ardèche, in the south-west of France. (_After Scrope_) 106 - - 29. Portion of a basaltic column from the Giant's Causeway, - exhibiting both the ball-and-socket and the - tenon-and-mortise structure 107 - - 30. Vein of green pitchstone at Chiaja di Luna, in the Island - of Ponza, breaking up into regular columns and into - spherical masses with a concentric series of joints. - (_After Scrope_) 108 - - 31. Illustration of the 'perlitic structure' in glassy rocks 109 - - 32. Transverse section of a lava-stream 111 - - 33. The Kammerbühl, or Kammerberg, Bohemia (as seen from - the south-west) 113 - - 34. Section of the Kammerbühl in Bohemia 114 - - 35. Natural section of a volcanic cone in the Island of Vulcano 116 - - 36. Section in the side of the Kammerbühl, Bohemia 118 - - 37. Experimental illustration of the mode of formation of - volcanic cones, composed of fragmental materials 120 - - 38. Natural section of a tuff-cone, forming the Cape of Misenum, - and exhibiting the peculiar internal arrangement, - characteristic of volcanoes composed of fragmentary - materials. (_After Scrope_) 121 - - 39. Section of a small scoria-cone formed within the crater of - Vesuvius in the year 1835, illustrating the filling up of - the central vent of the cone by subsequent ejections. - (_After Abich_) 122 - - 40. Volcanic cones composed of scoriæ, and breached on one - side by the outflow of lava-currents. (_After Scrope_) 128 - - 41. Campo Bianco, in the Island of Lipari. A pumice-cone - breached by the outflow of an obsidian lava-stream - _to face p._ 124 - - 42. Volcanic cones in Auvergne, which have suffered to some - extent from atmospheric denudation. (_After Scrope_) 124 - - 43. Experimental illustration of the mode of formation of - volcanic cones composed of viscid lavas. (_After Reyer_) 126 - - 44. The Grand Puy of Sarcoui, composed of trachyte, rising - between two breached scoria-cones (Auvergne). (_After - Scrope_) 126 - - 45. Volcanic cone (Mamelon) composed of very viscid lava - (Island of Bourbon). (_After Bory de St. Vincent_) 127 - - 46. Another Mamelon in the Island of Bourbon, with a crater - at its summit. (_After Bory de St. Vincent_) 127 - - 47. Cliff-section in the Island of Madeira, showing how a - composite volcano is built up of lava-streams, beds of - scoriæ, and dykes. (_After Lyell_) 125 - - 48. Section seen at the cascade, Bains du Mont Dore. (_After - Scrope_) 130 - - 49. Section in the Island of Ventotienne, showing a great - stream of andesitic lava overlying stratified tuffs. - (_After Scrope_) 130 - - 50. Cliff on the south side of the Island of San Stephano 131 - - 51. The headland of Monte della Guardia, in the Island of Ponza 131 - - 52. Western side of the same headland, as seen from the north - side of Luna Bay 132 - - 53. Sea-cliff at Il Capo, the north-east point of Salina, - showing stratified agglomerates traversed by numerous - dykes, the whole being unconformably overlaid by - stratified, aqueous deposits 137 - - 54. Section observed in the Val del Bove, Etna, showing a - basaltic dyke, from the upper part of which a - lava-current has flowed 138 - - 55. Basaltic dykes projecting from masses of stratified scoriæ - in the sides of the Val del Bove, Etna 134 - - 56. Sheets of igneous rock (basalt) intruded between beds of - sandstone, clay, and limestone (Island of Skye) 137 - - 57. Plan of the dissected volcano of Mull in the Inner - Hebrides _to face p._ 142 - - 58. Section of the volcano of Mull along the line A B " 142 - - 59. Summit of the volcano of Monte Sant' Angelo, in Lipari, - exhibiting a crater with walls worn down by denudation 158 - - 60. Outlines of lava-cones 160 - - 61. Diagram illustrating the formation of parasitic cones along - lines of fissure formed on the flanks of a great volcanic - mountain 162 - - 62. Outline of Etna, as seen from Catania 162 - - 63. Outline of Etna, as seen from the Val del Bronte 163 - - 64. Plan of the volcano forming the Island of Ischia 163 - - 65. A primary parasitic cone, with a secondary one at its - base--Ischia 164 - - 66. Scoria-cone near Auckland, New Zealand, with a lava-current - flowing from it. (_After Heaphy_) 165 - - 67. Section of rocks below the ancient triassic volcano of - Predazzo in the Tyrol 165 - - 68. Cotopaxi, as seen from a distance of ninety miles. (_After - Humboldt_) 168 - - 69. Citlaltepetl, or the Pic d'Orizaba, in Mexico, as seen from - the Forest of Xalapa. (_After Humboldt_) 169 - - 70. Lac Paven, in the Auvergne. (_After Scrope_) 171 - - 71. The crater-lake called Lago del Bagno, in Ischia, converted - into a harbour 172 - - 72. Lake of Gustavila, in Mexico. (_After Humboldt_) 172 - - 73. Peak of Teneriffe, surrounded by great crater-rings. (_After - Piazzi-Smyth_) 175 - - 74. The volcano of Bourbon, rising in the midst of a crater-ring - four miles in diameter. (_After Bory de St. Vincent_) 176 - - 75. The volcano of Bourbon, as seen from another point of - view, with three concentric crater-rings encircling its - base. (_After Bory de St. Vincent_) 176 - - 76. Vesuvius as seen from Sorrento, half encircled by the - crater-ring of Somma 177 - - 77. Outlines of various volcanoes illustrating the different - relations of the craters to cones _to face p._ 178 - - 78. Island thrown up In the Mediterranean Sea in July and - August, 1831. (_After the Prince de Joinville_) 179 - - 79. Sinter-cones surrounding the orifices of geysers 183 - - 80. Diagram illustrating the mode of formation of travertine- - and sinter-terraces on the sides of a hill of tuff 185 - - 81. Map of the volcanic group of the Lipari Islands, illustrating - the position of the lines of fissure upon which - the volcanoes have been built up 192 - - 82. The Puy de Pariou, in the Auvergne, illustrating the shifting - of eruption along a line of fissures 193 - - 83. Ideal section of the Puy de Pariou 194 - - 84. Fissure formed on the flanks of Etna during the emotion - of 1865. (_After Silvestri_) 194 - - 85. Plan of the Island of Vulcano, based on the map of the - Italian Government 196 - - 86. Vulcanello, with its three craters 197 - - 87. Section of basalt from Ovifak, Greenland, with particles of - metallic iron diffused through its mass 319 - - 88. Diagram illustrating the relations between the terrestrial - and the extra-terrestrial rocks _to face p._ 322 - - 89. A group of sun-spots. (_After Secchi_) 362 - - 90. A sun-spot, showing the great masses of incandescent - vapour rising or falling within it. (_After Secchi_) 363 - - 91. The edge of a sun-spot, showing a portion of the prominent - masses of incandescent gas (A) which detached itself - at B and floated into the midst of the cavity. - (_After Norman Lockyer_) 363 - - 92. Drawing of a solar prominence made by Mr. Norman - Lockyer, March 14, 1869, at 11 h. 5 m. A.M. 364 - - 93. The same object, as seen at 11 h. 15 m. on the same day. - (_After Norman Lockyer_) 365 - - 94. Drawings of a solar prominence at four different periods - on September 7, 1871. (_After Young_) 366 - - 95. A group of Lunar craters (Maurolycus, Barocius, &c.), the - largest being more than sixty miles in diameter 368 - - - - -VOLCANOES. - - - - -CHAPTER I. - -INTRODUCTORY: NATURE OF THE INQUIRY. - - -'What is a volcano?' This is a familiar question, often addressed to us -in our youth, which 'Catechisms of Universal Knowledge,' and similar -school manuals, have taught us to reply to in some such terms as the -following: 'A volcano is a burning mountain, from the summit of which -issue smoke and flames.' Such a statement as this, it is probable, does -not unfairly represent the ideas which are, even at the present day, -popularly entertained upon the subject. - -But in this, as in so many other cases, our first step towards the -acquirement of scientific or exact knowledge, must be the unlearning of -what we have before been led to regard as true. The description which -we have quoted is not merely incomplete and inadequate as a whole, -but each individual proposition of which it is made up is grossly -inaccurate, and, what is worse, perversely misleading. In the first -place, the action which takes place at volcanoes is not 'burning,' or -combustion, and bears, indeed, no relation whatever to that well-known -process. Nor are volcanoes necessarily 'mountains' at all; essentially, -they are just the reverse--namely, holes in the earth's crust, or -outer portion, by means of which a communication is kept up between -the surface and the interior of our globe. When mountains do exist at -centres of volcanic activity, they are simply the heaps of materials -thrown out of these holes, and must therefore be regarded not as the -causes but as the consequences of the volcanic action. Neither does -this action always take place at the 'summits' of volcanic mountains, -when such exist, for eruptions occur quite as frequently on their sides -or at their base. That, too, which popular fancy regards as 'smoke' -is really condensing steam or watery vapour, and the supposed raging -'flames' are nothing more than the glowing light of a mass of molten -material reflected from these vapour clouds. - -It is not difficult to understand how these false notions on the -subject of volcanic action have come to be so generally prevalent. -In the earlier stages of its development, the human mind is much -more congenially employed in drinking in that which is marvellous -than in searching for that which is true. It must be admitted, too, -that the grand and striking phenomena displayed by volcanoes are -especially calculated to inspire terror and to excite superstition, -and such feelings most operate in preventing those close and accurate -observations which alone can form the basis of scientific reasoning. - -[Sidenote: IDEAS OF THE ANCIENTS.] - -The ancients were acquainted only with the four or five active -volcanoes in the Mediterranean area; the term 'volcano' being the -name of one of these (Vulcano, or Volcano, in the Lipari Islands), -which has come to be applied to all similar phenomena. It is only in -comparatively modern times that it has become a known £act that many -hundreds of volcanoes exist upon the globe, and are scattered over -almost every part of its surface. Classical mythology appropriated -Vulcano as the forge of Hephæstus, and his Roman representative Vulcan, -while Etna was regarded as formed by the mountains under which a -vengeful deity had buried the rebellious Typhon; it may be imagined, -therefore, that any endeavour to more closely investigate the phenomena -displayed at these localities would be regarded, not simply as an act -of temerity, but as one of actual impiety. In mediæval times similar -feelings would operate with not less force in the same direction, for -the popular belief identified the subterranean fires with a place of -everlasting torment; Vulcano was regarded as the place of punishment -of the Arian Emperor Theodosius, while Etna was assigned to poor Anne -Boleyn, the perverter of faith in the person of its stoutest defender. -That such feelings of superstitious terror in connection with volcanoes -are, even at the present day, far from being extinct, will be attested -by every traveller who, in carrying on investigations about volcanic -centres, has had to avail himself of the assistance of guides and -attendants from among the common people. - -Among the great writers of antiquity we find several who had so far -emancipated their minds from the popular superstitions as to be able -to enunciate just and rational views upon the subject of volcanoes. -Until quite recent times, however, their teaching was quite forgotten -or neglected, and the modern science of Vulcanology may be said to have -entirely grown up within the last one hundred years. - -The great pioneer in this important branch of research was the -illustrious Italian naturalist Spallanzani, who, in the year 1788, -visited the several volcanoes of his native land, and published an -account of the numerous valuable and original observations which he -had made upon them. About the same time the French geologist Dolomieu -showed how much light might be thrown on the nature of volcanic action -by a study of the various materials which are ejected from volcanic -vents; while our own countryman. Sir William Hamilton, was engaged in -a systematic study of the changes in form of volcanic mountains, and -of the causes which determine their growth. At a somewhat later date -the three German naturalists. Von Buch, Humboldt, and Abich, greatly -extended our knowledge of volcanoes by their travels in different -portions of the globe. - -[Sidenote: CHARACTER OF MODERN RESEARCHES.] - -The first attempt, however, to frame a satisfactory theory of volcanic -action, and to show the part which volcanoes have played in the past -history of our globe, together with their place in its present economy, -was made in 1825, by Poulett Scrope, whose great work, 'Considerations -on Volcanoes,' may be regarded as the earliest systematic treatise on -Vulcanology. Since the publication of this work, many new lines of -inquiry have been opened up in connection with the subject, and fresh -methods of research have been devised and applied to it. More exact -observations of travellers over wider areas have greatly multiplied -the facts upon which we may reason and speculate, and many erroneous -hypotheses which had grown up in connection with the subject have been -removed by patient and critical inquiry. - -We propose in the following pages to give an outline of the present -state of knowledge upon the subject, and to indicate the bearings -of those conclusions which have already been arrived at, upon the -great questions of the history of our globe and the relations which -it bears to the other portions of the universe. In attempting this -task we cannot do better than take up the several lines of inquiry -in the order in which they have been seized upon and worked out by -the original investigators; for never, perhaps, is the development -of thought in the individual mind so natural in its methods, and so -permanent in its effects, as when it obeys those laws which determined -its growth in the collective mind of the race. In our minds, as in our -bodies, development in the individual is an epitome, or microcosmic -reproduction, of evolution in the species. - - - - -CHAPTER II. - -THE NATURE OF VOLCANIC ACTION. - - -The dose investigation of what goes on within a volcanic vent may -appear at first sight to be a task beset with so many difficulties and -dangers that we may be tempted to abandon it as altogether hopeless. At -the first recorded eruption of Vesuvius the elder Pliny lost his life -in an attempt to approach the mountain and examine the action which -was taking place there; and during the last great outburst of the same -volcano a band of Neapolitan students, whose curiosity was greater than -their prudence, shared the same fate. - -But in both these cases the inquirers paid the penalty of having -adopted a wrong method. If we wish to examine the mode of working of a -complicated steam-engine, it will be of little avail for us to watch -the machinery when the full blast of steam is turned on, and the rapid -movements of levers, pinions, and slides baffle all attempts to follow -them, and render hopeless every effort to trace their connection with -one another. But if some friendly hand turn off the greater part of -the steam-supply, then, as the rods move slowly backwards and forwards, -as the wheels make their measured revolutions, and the valves axe -seen gradually opening and shutting, we may have an opportunity of -determining the relations of the several parts of the machine to one -another, and of arriving at just conclusions concerning the plan on -which it is constructed. Nor can we doubt that the parts of the machine -bear the same relation to one another, and that their movements take -place in precisely the same order, when the supply of steam is large as -when it is small. - -Now, as we shall show in the sequel, a volcano is a kind of great -natural steam-engine, and our best method of investigating its action -is to watch it when a part of the steam-supply is cut off. It is -true that we cannot at will control the source of supply of steam to -a volcano, as we can in a steam-engine, but as some volcanoes have -usually only a small steam-supply, and nearly all volcanoes vary -greatly in the intensity of their action at different periods, we can, -by a careful selection of the object or the time of our study, gain all -those advantages which would be obtained by regulating its action for -ourselves. - -Spallanzani appears to have been the first to perceive the important -fact, that the nature of volcanic action remains the same, however -its intensity may vary. Taking advantage of the circumstance that -there exists in the Mediterranean Sea a volcano--Stromboli--which -for at least 2,000 years has been in a constant and regular, but not -in a violent or dangerous, state of activity, he visited the spot, -and made the series of careful observations which laid the foundation -of our knowledge of the 'physiology of volcanoes.' Since the time -of Spallanzani, many other investigators have visited the crater -of Stromboli, and they have been able to confirm and extend the -observations of the great Italian naturalist, as to the character of -the action which is constantly taking place within it. We cannot better -illustrate the nature of volcanic action than by describing what has -been witnessed by numerous observers within the crater of Stromboli, -where it is possible to watch the series of operations going on by the -hour together, and to do so without having our judgment warped either -by an excited imagination or the sense of danger. - -[Sidenote: APPEARANCE OF STROMBOLI FROM A DISTANCE.] - -In the sketch, fig. 1, which was made on April 20, 1874, I have shown -the appearance which this interesting volcano usually presents, when -viewed from a distance. The island is of rudely circular outline, and -conical form, and rises to the height of 3,090 feet above the level of -the Mediterranean. From a point on the side of the mountain, masses -of vapour are seen to issue, and these unite to form a cloud over -the mountain, the outline of this vapour-cloud varying continually -according to the hygrometric state of the atmosphere, and the direction -and force of the wind. At the time when this sketch was made, the -vapour-cloud was spread in a great horizontal stratum overshadowing -the whole island, but it was clearly seen to be made up of a number -of globular masses, each of which, as we shall hereafter see, is the -product of a distinct outburst of the volcanic forces. - -Viewed at night-time, Stromboli presents a far more striking and -singular spectacle. The mountain, with its vapour canopy, is visible -over an area having a radius of more than 100 miles. When watched from -the deck of a vessel anywhere within this area, a glow of red light is -seen to make its appearance from time to time above the summit of the -mountain; this glow of light may be observed to increase gradually in -intensity, and then as gradually to die away. After a short interval -the same appearances are repeated, and this goes on till the increasing -light of the dawn causes the phenomenon to be no longer visible. The -resemblance presented by Stromboli to a 'flashing light' on a most -gigantic scale is very striking, and the mountain has long been known -as 'the lighthouse of the Mediterranean.' - -It must be pointed out, however, that in two very important particulars -the appearances presented by Stromboli differ markedly from those -rhythmical gleams exhibited by the 'flashing-lights' of our coasts. -In the first place, the intervals between successive flashes are very -unequal, varying from less than one minute to twenty minutes, or even -more; and in the second place, the duration and intensity of the red -glow above the mountain are subject to like variation, being sometimes -a momentary scarcely visible gleam, and at others a vivid burst of -light which illuminates the sky to a considerable distance round. - -[Illustration: Fig. 1.--Stromboli, viewed from the North-west, April -1874.] - -[Illustration: Fig. 2.--Map of tub Island of Stromboli. (Scale about -two inches to a mile.)] - -[Sidenote: GENERAL FEATURES OF THE MOUNTAIN.] - -Let us now draw near and examine this wonderful phenomenon of a -mountain which seemingly ever burns with fire, and yet is not consumed. -The general form of the Island of Stromboli will be gathered from an -inspection of the plan, fig. 2, which is copied from a map published by -the Italian Government. When we land upon the island, we find that it -is entirely built up of such materials as we know to be ejected from -volcanoes; indeed, it resembles on a gigantic scale the surroundings -of an iron furnace, with its heaps of cinders and masses of slag. The -irregularity in the form of the island is at once seen to be due to the -action of the wind, the rain, and the waves of the surrounding sea, -which have removed the loose, cindery materials at some points, and -left the hard, slaggy masses standing up prominently at others. - -This great heap of cindery and slaggy materials rises, as we have said, -to a height of more than 3,000 feet above the sea-level, but even this -measurement does not give a just idea of its vast bulk. Soundings in -the sea surrounding the island show that the bottom gradually shelves -around the shores to the depth of nearly 600 fathoms, so that Stromboli -is a great conical mass of cinders and slaggy materials, having a -height of over 6,000 feet, and a base whose diameter exceeds four miles. - -The general form and proportions of this mass will be better understood -by an examination of the section, fig. 3, which is also constructed -from the materials furnished by the map of the island issued by the -Italian Government. The same section, and the map, fig. 2, will serve -to make clear the position and relations of the point on the mountain -at which the volcanic activity takes place. At a spot on the north-west -slope of the mountain, about 1,000 feet below its summit, and 2,000 -feet above the level of the sea, there exists a circular depression, -the present active 'crater' of the volcano; and leading down from this -to the sea there is a flat slope making an angle of about 35° with the -horizon, and known as the 'Sciarra.' The Sciarra is bounded by steep -cliffs, as shown in the sketch fig. 1, and the plan fig. 2. - -[Illustration: Fig. 3.--Section through the Island of Stromboli from -n.w. to s.e. - -_a._ Highest summit of the mountain, _c._ Cratère del Fossa, _b._ Point -overlooking the crater, _d._ Steep slope known as the Sciarra del -Fuoco. _e._ Continuation of the same slope beneath the level of the -sea. _f._ Steep cliffs of the Punta dell' Omo.] - -[Sidenote: FORM AND FUNCTION OF THE CRATER.] - -If we climb up to this scene of volcanic activity, we shall be able to -watch narrowly the operations which are going on there. On the morning -of the 24th of April, 1874, I paid a visit to this interesting spot in -order to get a near view of what was taking place. On reaching a point -upon the side of the Sciarra, from which the crater was in full view -before me, I witnessed, and made a sketch of, an outburst which then -took place, and this sketch has been reproduced in fig. 4. Before the -outburst, numerous light curling wreaths of vapour were seen ascending -from fissures on the sides and bottom of the crater. Suddenly, and -without the slightest warning, a sound was heard like that produced -when a locomotive blows off its steam at a railway-station; a great -volume of watery vapour was at the same time thrown violently into -the atmosphere, and with it there were hurled upwards a number of -dark fragments, which rose to the height of 400 or 500 feet above the -crater, describing curves in their course, and then falling back upon -the mountain. Most of these fragments tumbled into the crater with a -loud, rattling noise, but some of them fell outside the crater, and a -few rolled down the steep slope of the Sciarra into the sea. Some of -these falling fragments were found to be still hot and glowing, and in -a semi-molten condition, so that they readily received the impression -of a coin thrust into them. - -[Illustration: Fig. 4.--The Crates of Stromboli as viewed from the side -of the Sciarra during an eruption on the morning of April 24, 1874.] - -[Sidenote: APERTURES AT THE BOTTOM OF THE CRATER.] - -But on the upper side of the crater, at the point marked 6, on the -section fig. 3, there exists a spot from which we can look down upon -the bottom of the crater, and view the operations taking place there. -This is the place where Spallanzani and other later investigators have -carried on their observations, and, when the wind is blowing from -the spectator towards the crater, he may sit for hours watching the -wonderful scene displayed before him. The black slaggy bottom of the -crater is seen to be traversed by many fissures or cracks, from most -of which curling jets of vapour issue quietly, and gradually mingle -with and disappear in the atmosphere. But besides these smaller cracks -at the bottom of the crater, several larger openings are seen, which -vary in number and position at different periods; sometimes only one of -these apertures is visible, at others as many as six or seven, and the -phenomena presented at these larger apertures are especially worthy of -careful investigation. - -These larger apertures, if we study the nature of the action taking -place at them, may be divided into three classes. From those of the -first class, steam is emitted with loud, snorting puffs, like those -produced by a locomotive-engine, but far less regular and rhythmical -in their succession. In the second class of apertures masses of molten -material are seen welling out, and, if the position of the aperture -be favourable, flowing outside the crater; from this liquid molten -mass steam is seen to escape, sometimes in considerable quantities. -The openings of the third class present still more interesting -appearances. Within the walls of the aperture a viscid or semi-liquid -substance is seen slowly heaving up and down. As we watch the seething -mass the agitation within it is observed to increase gradually, and at -last a gigantic bubble is formed which violently bursts, when a great -rush of steam takes place, carrying fragments of the scum-like surface -of the liquid high into the atmosphere. - -If we visit the crater by night, the appearances presented are found to -be still more striking and suggestive. The smaller cracks and larger -openings glow with a ruddy light. The liquid matter is seen to be red- -or even white-hot, while the scum or crust which forms upon it is of a -dull red colour. Every time a bubble bursts and the crust is broken up -by the escape of steam, a fresh, glowing surface of the incandescent -material is exposed. If at these moments we look up at the vapour-cloud -covering the mountain, we shall at once understand the cause of the -singular appearances presented by Stromboli when viewed from a distance -at night, for the great masses of vapour are seen to be lit up with a -vivid, ruddy glow, like that produced when an engine-driver opens the -door of the furnace and illuminates the stream of vapour issuing from -the funnel of his locomotive. - -Let us now endeavour to analyse the phenomena so admirably displayed -before us in the crater of Stromboli. The three essential conditions -on which the production of these phenomena seems to depend are the -following: first, the existence of certain apertures or cracks -communicating between the interior and the surface of the earth; -secondly, the presence of matter in a highly heated condition beneath -the surface; and thirdly, the existence of great quantities of water -imprisoned in the subterranean regions--which water, escaping as steam, -gives rise to all those active phenomena we have been describing. - -[Sidenote: CAUSE OF THE GLOWING LIGHT.] - -We have said, at the outset, that there exists no analogy whatever -between the action which takes place in volcanoes and the operation of -burning or combustion. Occasionally, it is true, certain inflammable -substances are formed by the action going on within the volcano, and -these inflammable substances, taking fire, produce real flames. Such -flames are, however, in almost all cases only feebly luminous, and do -not give rise to any conspicuous appearances. What is usually taken for -flame during volcanic eruptions is simply, as we have already pointed -out, the glowing red-hot surface of a mass of molten rock, reflected -from a vapour-cloud hanging over it. The red glow observed over a -volcano in eruption is indeed precisely similar in its nature and -origin to that which is seen above London during a night of heavy fog, -and which is produced by the reflection of the gas-lights of the city -from the innumerable particles of water-vapour diffused through the -atmosphere. Fires, of course, occur when the molten and incandescent -materials poured out from a volcano come in contact with inflammable -substances, such as forests and houses, but in these cases the -combustion is quite a secondary phenomenon. - -There is another popular delusion concerning volcanic action, which it -may be necessary to refer to and to combat. From the well-known fact -that sulphur or brimstone is found abundantly in volcanic regions, the -popular belief has arisen that this highly inflammable substance has -something to do with the production of the eruptions of volcanoes. In -school-books which were, until comparatively recent years, in constant -use in this country, the statement may be found that by burying certain -quantities of sulphur, iron-pyrites, and charcoal in a hole in the -ground, we may form a miniature volcano, and produce all the essential -phenomena of a volcanic eruption. No greater mistake could possibly be -made. The chemical reactions which take place when sulphur and other -substances are made to act upon each other differ entirely from the -phenomena of volcanic action. The sulphur which is found in volcanic -regions is the result and not the cause of volcanic action. Among the -most common substances emitted from volcanic vents along with the steam -are the two gases, sulphurous acid and sulphuretted hydrogen. When -these two gases come into contact with one another, chemical action -takes place, and the elements contained in them--oxygen, hydrogen, -and sulphur--are free to group themselves together in an entirely new -fashion; the consequence of this is that water and sulphuric acid (oil -of vitriol) are formed, and a certain quantity of sulphur is set free. -The water escapes into the atmosphere, the sulphuric acid combines with -lime, iron, or other substances contained in the surrounding rocks, -and the sulphur builds up crystals in any cavities which may happen to -exist in these rocks. - -[Sidenote: VOLCANIC ACTION RESEMBLES BOILING.] - -If, however, careful and exact observations, like those carried on -at Stromboli, compel us to reject the popular notions concerning -the supposed resemblance between volcanic action and the combustion -of sulphur or other substances, they nevertheless suggest analogies -with certain other simple and well-known operations. And in pursuing -these analogies, we are led to the recognition of some admirable -illustrations both of the attendant phenomena and of the true cause of -volcanic outbursts. - -No one can look down on the mass of seething material in violent -agitation within the fissures at the bottom of the crater of Stromboli, -without being forcibly reminded of the appearances presented by liquids -in a state of boiling or ebullition. The glowing material seems to be -agitated by two kinds of movements, the one whirling or rotatory, the -other vertical or up-and-down in its direction. The fluid mass in this -way appears to be gradually impelled upwards, till it approaches the -lips of the aperture, when vast bubbles are formed upon its surface, -and to the sudden bursting of these the phenomena of the eruption are -due. - -Now if we take a tall narrow vessel and fill it with porridge or -some similar substance of imperfect fluidity, we shall be able, by -placing it over a fire, to imitate very closely indeed the appearances -presented in the crater of Stromboli. As the temperature of the mass -rises, steam is generated within it, and in the efforts of this steam -to escape, the substance is set in violent movement. These movements of -the mass are partly rotatory and partly vertical in their direction; as -fresh steam is generated in the mass its surface is gradually raised, -while an escape of the steam is immediately followed by a fall of the -surface. Thus an up-and-down movement of the liquid is maintained, but -as the generation of steam goes on faster than it can escape through -the viscid mass, there is a constant tendency in the latter to rise -towards the mouth of the vessel. At last, as we know, if heat continues -to be applied to the vessel, the fluid contents will be forced up to -its edge and a catastrophe will occur; the steam being suddenly and -violently liberated from the bubbles formed on the surface of the mass, -and a considerable quantity of the material forcibly expelled from -the vessel. The suddenness and violence of this catastrophe is easily -accounted for, if we bear in mind that the escaping steam acts after -the manner of a compressed spring which is suddenly released. Steam -is first formed at the bottom of the vessel which is in contact with -the fire; but here it is under the pressure of the whole mass of the -liquid, and moreover, the viscidity of the substance tends to retard -the union of the steam bubbles and their rise to the surface of the -mass. But when the pressure is relieved by the bursting of bubbles at -the surface, the whole of the generated steam tends to escape suddenly. - -[Sidenote: ESCAPE OF STEAM-BUBBLES FROM LAVA.] - - -Now within the crater of Stromboli we have precisely the necessary -conditions for the display of the same series of operations. In the -apertures at the bottom there exists a quantity of imperfectly fluid -materials at a higher temperature, containing water entangled in its -mass. As this water passes into the state of steam it tends to escape, -and in so doing puts the whole mass into violent movement. When the -steam rises to the surface, bubbles are formed, and the formation of -these bubbles is promoted by the circumstance that the liquid mass, -where exposed to the atmosphere, becomes chilled, and thereby rendered -less perfectly fluid. By the bursting of these bubbles the pressure is -partially relieved, and a violent escape of the pent-up steam takes -place through the whole mass. Equilibrium being thus restored, there -follows a longer or shorter interval of quiescence, during which steam -is being generated and collected within the mass, and the series of -operations which we have described then recommences. - -There is one other consideration which must be borne in mind in -connection with this subject. It is well known that if water be -subjected to sufficiently great pressure it may be raised to a very -high temperature and still retain its liquid condition. When this -pressure is removed, however, the whole mass passes at once into the -condition of steam or water-gas; and the gas thus formed at high -temperatures has a proportionably high tension. In a Papin's digester -water confined in a strong vessel is raised to temperatures far above -its ordinary boiling-point, and from any opening in such a vessel the -steam escapes with prodigious violence. Now, at considerable depths -beneath the earth's surface, and under the pressure of many hundreds -or thousands of feet of solid rock, water still retaining its liquid -condition may become intensely heated. When the pressure is relieved by -the formation of a crack or fissure in the superincumbent mass of rock, -the escape of the superheated steam will be of very violent character, -and may be attended with the most striking and destructive results. In -the existence of high temperatures beneath the earth's surface, and the -presence in the same regions of imprisoned water capable of passing -into the highly elastic gas which we call steam, we have a cause fully -competent to produce all the phenomena which we have described as -occurring at Stromboli. - -It may at first sight appear that the grand and terrible displays -of violence witnessed during a great volcanic eruption differ -fundamentally in their character and their origin from those feeble -outbursts which we are able to examine closely and analyse rigorously -at Stromboli. But that such is not the case a few simple considerations -will soon convince us. - -[Sidenote: STROMBOLI COMPARED WITH VESUVIUS.] - -Although Stromboli usually displays the subdued and moderate activity -which we have been describing, yet the intensity of the action going -on within it is subject to considerable variation. Occasionally the -violence of the outbursts is greatly increased--the roaring of the -steam-jets may be heard for many miles around, considerable streams of -incandescent liquefied rock flow down the Sciarra into the sea, and the -explosions in the crater are far more frequent and energetic, cinders -and fragments of rock being scattered all over the island and the -surrounding seas. - -On the other hand, volcanoes like Vesuvius, which are sometimes the -scene of eruptions on the very grandest scale, at others subside into a -temporary state of moderate activity quite similar in character to that -which is the normal condition of Stromboli. Thus, shortly before the -great eruption of Vesuvius in April 1872, a small cone was formed near -the edge of the crater, and during some months observers could watch, -in ease and safety, a series of small explosions taking place, quite -similar in their character and attendant phenomena to those which we -have described as occurring at Stromboli. French geologists are in the -habit of defining the condition of activity in a volcano by speaking -of the more quiet and, regular state as the 'Strombolian stage,' and -the more violent and paroxysmal as the 'Vesuvian stage'; but the two -conditions are, as we have seen, presented by the same volcano at -different periods, and pass into one another by the most insensible -gradations. - -We must now proceed to compare the grand and terrible appearances -presented during a great eruption with those more feeble displays which -we have been describing, to show that in all their essential features -these different kinds of outbursts are identical with one another, and -must be referred to the action of similar causes. - -The volcanic eruption which has been most carefully studied in recent -times is that which we have already referred to as occurring at -Vesuvius, in the month of April 1872. With the exception, perhaps, of -that which took place in October 1822, this eruption was the grandest -which has broken out at Vesuvius during the present century. Owing to -the circumstance of its proximity to the great city of Naples, Vesuvius -has always been the most carefully watched of all volcanoes, and in -recent years the erection of an observatory, provided with instruments -for recording the smallest subterranean tremors affecting the mountain, -has facilitated the carrying on of those continuous and minute -observations which are so necessary for exact scientific inquiry. - -[Illustration: Fig 5. Vesuvius in Eruption, as seen from Naples, April -26, 1872. (_From a photograph_)] - -[Sidenote: VESUVIUS ERUPTION OF 1872.] - -On the occasion of this outburst, the aid of instantaneous photography -was first made available for obtaining a permanent record of the -appearances displayed at volcanic eruptions. In fig. 5 we have one of -these photographs, which was taken at 5 o'clock P.M. on April 26, 1872, -transferred to a wood-block and engraved. In examining it we feel sure -that we are not being misled by any exaggeration or error on the part -of the artist. Vesuvius rises to the height of nearly 4,000 feet above -the level of the sea, and an inspection of the photograph proves that -the vapours and rock-fragments were thrown to the enormous height of -20,000 feet, or nearly four miles, into the atmosphere. - -The main features of this terrifying outburst were as follows. For -more than a twelvemonth before, the activity of the forces at work -within the mountain appeared to be gradually increasing, and the great -eruption commenced on April 24, attained its climax on the 26th, and -began to die out on the following day. During the eruption the bottom -of the crater was entirely broken up, and the sides of the mountain -were rent by fissures in all directions. So numerous were these -fissures and cracks that liquid matter appeared to be oozing from every -part of its surface, and, as Professor Palmieri, who witnessed the -outburst from the observatory, expressed it, 'Vesuvius sweated fire.' -One of the fissures was of enormous size, extending from the summit to -far beyond the base of the cone; the scar left by this gigantic rent -being plainly visible at the present day. - -From the great opening or crater at the summit, and from some of the -fissures on the sides of the mountain, enormous volumes of steam rushed -out with a prodigious roaring sound, the noise being so terrific that -the inhabitants of Naples, five miles off, fled from their houses -and spent the night in the open streets. Although this roaring sound -appeared at a distance to be continuous, yet those upon the mountain -could perceive that it was produced by detonations or explosions -rapidly following one another. Each of these explosions was accompanied -by the formation of a great globe of white vapour, which, rising into -the atmosphere, swelled the bulk of the vast cloud overhanging the -mountain. An inspection of the photographs (see fig. 5) shows that the -great vapour-cloud over Vesuvius was made up of the globular masses -ejected at successive explosions. Each of these explosive upward -rushes of steam carried along with it a considerable quantity of solid -fragments, and these fell in great numbers all over the surface of -the mountain, breaking the windows of the observatory, and making it -dangerous to be out of doors. - -We have said that lava, or molten rock, appeared to be issuing from -the very numerous cracks formed all over the flanks of the mountain. -But at three points this molten rock issued in such quantities as to -form great, fiery floods, which rushed down the sides of the mountain, -and flowed to a considerable distance beyond its base. The largest -of these lava-floods overwhelmed and destroyed the two villages of -Massa di Somma and San Sebastiano, besides many country houses in the -neighbourhood. - -[Sidenote: STEAM EMITTED FROM LAVA-CURRENT.] - -A very marked and interesting feature exhibited by these three -lava-floods was the quantity of watery vapour which they gave off -during their flow. All along their course, enormous volumes of steam -were evolved from them, as will be seen by an inspection of the -photograph. Indeed, such was the abundance and tension of the steam -thus escaping from the surfaces of the lava-currents that it forced -the congealing rock up into great bubbles and blisters, and gave rise -to the formation of innumerable miniature volcanoes, varying in size -from a beehive to a cottage, some of which remained in a state of -independent activity for a considerable time. - -So far, what we have described as taking place at Vesuvius, in April -1872, has been only the repetition on a £Eur grander scale of the -three kinds of action which we have shown to be constantly taking -place at Stromboli; namely, the formation of cracks or fissures in the -earth's surface, the escape of steam with explosive violence from these -openings, often propelling rock-fragments into the atmosphere, and the -outwelling, under the influence of this compressed steam, of masses of -molten materials. - -There were some other appearances presented at the great outburst at -Vesuvius, which do not seem at first sight to find any analogies in -the manifestations of the more feeble action continually going on at -Stromboli. - -Before and during the great outbreak of April 1872, Vesuvius itself -and the whole country round were visited with earthquake-shocks, or -tremblings of the ground. The sensitive instruments in the Vesuvian -Observatory showed the mountain daring the eruption to be in a constant -state of tremor. These earthquakes are not, as is commonly supposed, -actual upheavings of the earth's surface, but are vibrations propagated -through the solid materials of which the earth is built up. We cannot -stamp our feet upon the ground without giving rise to such vibrations, -though our senses may not be sufficiently acute to perceive them. -The explosive escape of steam from a crack is a cause sufficiently -powerful to produce a shock which is propagated and may be felt for -a considerable distance round. Even on Stromboli an observer at the -edge of the crater may notice that each explosive outburst of steam -is accompanied by a perceptible tremor of the ground, and in the case -of Vesuvius the violent shocks produced by the escape of far larger -volumes of steam give rise to proportionately stronger vibrations. The -nature and origin of those far more terrible and destructive shocks -which sometimes accompany, and more frequently precede, great volcanic -eruptions, we shall consider in the sequel. - -[Sidenote: CAUSE OF LIGHTNING DURING ERUPTIONS.] - -Another striking phenomenon which was exhibited in the great eruption -of Vesuvius in 1872 was the vivid display of lightning accompanied -by thunder. The uprushing current of steam and rock-fragments forms -a vertical column, but as the steam condenses it spreads out into a -great horizontal cloud which is seen to be made up of the great globes -of vapour emitted at successive explosions. When there is little or -no wind the vertical column with a horizontal cloud above it bears a -striking resemblance to the stone-pine trees which form so conspicuous -a feature in every Neapolitan landscape. Around this column of vapour -the most vivid lightning constantly plays and adds not a little to the -grand and awful character of the spectacle of a volcanic eruption, -especially when it is viewed by night. - -In the eruption of 1872 a strong wind blowing from the north-west -destroyed the usual regular appearance of this 'pine-tree appendage' to -the mountain, which is so well known to, and dreaded by the inhabitants -of Naples; the cloud, as will be seen from the photograph (fig. 5, -_facing_ p. 24), was blown on one side, and most of the falling -fragments took the same direction. - -It is well known that when high-pressure steam is allowed to escape -through an orifice, electricity is abundantly generated by the -friction, and Sir William Armstrong's hydro-electric machine is -constructed on this principle. Every volcano in violent eruption is -a very efficient hydro-electric machine, and the uprushing column -is in a condition of intense electrical excitation. This result is -probably aided by the friction of the solid particles as they are -propelled upwards and fall back into the crater. The restoration of -the condition of electrical stability between this column and the -surrounding atmosphere is attended with the production of frequent -lightning-flashes and thunder-claps, the found of the latter being -usually, however, drowned in the still louder roar of the uprushing -steam-column. - -The discharge of Buch large quantities of steam into the atmosphere -soon causes the latter to be saturated with watery vapour, and there -follows an excessive rainfall; long-continued rain and floods were an -accompaniment of the great Vesuvian outbreak of 1872, as they have -been of almost all great volcanic eruptions. The Italians, indeed, -dread the floods which follow an eruption more than the fiery streams -of lava which accompany it--for they have found the mud-streams (_lave -di fango_), formed by rain-water sweeping along the loose volcanic -materials, to be more widely destructive in their effects than the -currents of molten rock (_lave di fuoco_). - -Besides the phenomena which we have now described as accompanying a -great volcanic outburst, many others have undoubtedly been recorded -by apparently trustworthy authorities. But, in dealing with the -descriptions of these grand and terrible events, we must always be on -our guard against accepting as literal facts, the statements made by -witnesses, often writing at some distance from the scene of action, and -almost always under the influence of violent excitement and terror. -The desire to administer to the universal love of the marvellous, and -the tendency to exaggeration, will usually account for many of the -wonderful statements contained in such records; and, even where the -witness is accurately relating events which he thinks passed before his -eyes, we must remember that it is probable he may have had neither the -opportunity nor the capacity for exact observation. - -The more carefully we sift the accounts which have been preserved of -great volcanic outbursts, the more are we struck by the fact that the -appearances described can be resolved into a few simple operations, the -true character of which has been distorted or disguised by the want of -accurate observation on the part of the witnesses. - -[Sidenote: SIMILARITY OF FEEBLE AND VIOLENT ERUPTIONS.] - -We are thus led to the conclusion that the grand and terrible -appearances displayed at Vesuvius and other volcanoes in a state of -violent eruption do not differ in any essential respect from the -phenomena which we have witnessed accompanying the miniature outbursts -of Stromboli. And we are convinced, by the same considerations, that -the forces which give rise to the feeble displays in the latter case -would produce, if acting with greater intensity and violence, all the -magnificent spectacles presented in the former. - -In Vesuvius and Stromboli alike, the active cause of all the phenomena -exhibited is found to be the escape of steam from the midst of -masses of incandescent liquefied rock. The violence, and therefore -the grandeur and destructive effects of an eruption, depend upon the -abundance and tension of this escaping steam. - -There is one respect in which volcanic phenomena are especially -calculated to excite the fear and wonder of beholders--namely, in the -sudden and apparently spontaneous character of their manifestations. -Eclipses were regarded as equally portentous with volcanic eruptions -till astronomers learned not only to explain the causes which gave -rise to them, but even to predict to the second the times of their -occurrence. If we were able in like manner to warn the inhabitants of -volcanic regions of the approach of a grand eruption, the fear and -superstition with which these events are now regarded would doubtless -be in great part dispelled. The power of prediction is alike the -crucial test and the crowning triumph of a scientific theory. - -But, although natural philosophers are able to assign the causes to -which the grand operations of volcanoes are due, and also to explain -all the varied appearances which accompany them, they have not as yet -so far mastered the laws which govern volcanic action as to be able to -predict the periods of their manifestation. - -That these operations, like all others going on upon the globe, are -governed by great natural laws we cannot for a moment doubt. And that, -in all probability, more careful and exact observation and reasoning -will at some future time lead us to the recognition of these laws, -every student of nature is sanguine. But at the present time, it must -be confessed, we are very far indeed from being able to afford that -crowning proof of the truth of our theories of volcanic action which is -implied in the power of predicting the period and degree of intensity -of their manifestations. - -[Sidenote: ERUPTIONS AND THE INTERVALS BETWEEN THEM.] - -There are, however, some observations which lead us to hope that the -time may not be far distant when we shall have so £Eur obtained a -knowledge of the conditions on which volcanic action depends as to be -able to form some judgment as to its manifestations in the future at -any particular locality. But we must recollect that these conditions -axe very numerous and complicated, and that some of them may lie almost -entirely outside our sphere of observation; hence hasty attempts in -this direction, such as have recently been made, are to be deprecated -by every true lover of science. - -Concerning the eruptions that have taken place at those volcanic -centres which have been known from a remote antiquity, we have records -from which we can determine the intervals separating these outbursts -and their relative violence. A critical examination of these records -leads to the following conclusions:-- - -(1.) A long period of quiescence is generally followed by an eruption -which is either of long duration or of great violence. - -(2.) A long-continued, or very violent eruption is usually followed by -a prolonged period of repose. - -(3.) Feeble and short eruptions usually succeed one another at brief -intervals. - -(4.) As a general rule, the violence of a great eruption is inversely -proportional to its duration. - -It will be seen that these general conclusions are in perfect harmony -with the theory that volcanic outbursts are due to the accumulation -of steam at volcanic centres, and that the tension of this imprisoned -gas eventually overcomes the repressing forces which tend to prevent -its manifestation. Before astronomers had learnt to determine all -the conditions on which the production of eclipses depends, they had -found that these phenomena succeed one another at regular intervals. -The discovery of such astronomical cycles was a great advance in our -knowledge of the heavenly bodies, and in the same way the determination -of these general relations between the intensity and duration of -volcanic outbursts and the intervals of time which separate them may -be regarded as the first step towards the discovery of the laws which -govern volcanic activity. - -In the actual determination of the conditions upon which the occurrence -of volcanic eruptions depends, it must be confessed, however, that -very little has as yet been done. This is in part due to the fact that -some at least of these conditions lie beyond the limits of direct -observation. But it must also be admitted, on the other hand, that -little has been as yet accomplished towards the careful and systematic -observation of those phenomena which may, and probably do, exert an -influence in bringing about volcanic outbursts. - -[Sidenote: INFLUENCE OF ATMOSPHERIC CONDITIONS.] - -In the Lipari Islands there has prevailed a belief, from the very -earliest period of history, that the feeble eruptions of Stromboli -are in some way dependent upon the condition of the atmosphere. -These islands were known to the ancients as the Æolian Isles, from -the fact that they were once ruled over by a king of the name of -Æolus. It seems not improbable that Æolus was gifted with natural -powers of observation and reasoning far in advance of those of his -contemporaries. A careful study of the vapour-cloud which covers -Stromboli would certainly afford him information concerning the -hygrometric condition of the atmosphere; the form and position -assumed by this vapour-cloud would be a no less perfect index of the -direction and force of the wind; and, if the popular belief be well -founded, the frequence and violence of the explosions taking place -from the crater would indicate the barometric pressure. From these -data an acute observer would be able to issue 'storm-warnings' and -weather-prognostics of considerable value. In the vulgar mind, the -idea of the prediction of natural events is closely bound up with that -of their production; and the shrewd weather-prophet of Lipari was -after his death raised to the rank of a god, and invested with the -sovereignty of the winds. - -Whether the popular idea that the outbursts of Stromboli are regulated -by atmospheric conditions has any foundation is still open to grave -doubt. It seems to be certain, however, that during autumn and winter -the more violent paroxysms of the volcano occur, and that in summer -the action which takes place is far more regular and equable. It would -be of the greatest benefit to science if an observatory were erected -beside the crater of Stromboli, where a careful record might be kept of -all atmospheric changes, and of the synchronous manifestations of the -volcanic forces. - -A little consideration will show that it is a by no means unreasonable -supposition that variations in atmospheric pressure may exercise a very -important influence in bringing about volcanic outbursts. Changes in -the barometer to the extent of two inches within a very short period -are not uncommon occurrences. A very simple calculation will show that -the fall of the mercury in the barometer to the extent of two inches -indicates the removal of a weight of two millions of tons from each -square mile of the earth's surface where this change takes place. Now, -if we suppose, as we have good ground for doing, that under volcanic -areas vast quantities of superheated water are only prevented from -flashing into steam by the superincumbent pressure, a relief of this -pressure to the extent of two millions of tons on every square mile -could scarcely fail to produce very marked effects. The way in which -explosions in fiery coal-mines generally follow closely upon sudden -falls in the atmospheric pressure is now well known; and coal-mine -explosions and volcanic outbursts have this in common, that both -result from the sudden and violent liberation of subterranean gases. -There are not a few apparently well-authenticated accounts of volcanic -and earthquake phenomena following closely on peculiar atmospheric -conditions, and the whole question of the relation of the volcanic -forces to atmospheric pressure, as Spallanzani himself so long ago -pointed out, is deserving of a most careful and rigorous investigation. - -[Sidenote: SUPPOSED TIDAL EFFECTS.] - -There is one other consideration which has frequently been urged as -worthy of especial attention, in dealing with the question of the -exciting causes of volcanic outbursts. If volcanoes were, as was at one -time almost universally supposed, in direct communication with a great -central mass of liquefied materials, or even if any large reservoirs -of such liquids existed beneath volcanic districts, as others have -imagined, then the different mobility of the solid and liquid portions -of the earth's mass would give rise to tidal effects similar to those -occurring in the surface waters of the globe. Under such circumstances, -volcanic outbursts, like the tides, would be determined by the relative -positions of the sun and moon to our globe. It is certain, however, -that no very direct relation has yet been established between the -lunar periods and those of volcanic outbursts, though recent close -observations upon the crater of Vesuvius, by Professor Palmieri, do -seem to lend support to the view that such relations may exist. - -At the present time, therefore, it must be admitted that vulcanologists -have only just commenced those series of exact and continuous -observations which are necessary to determine the conditions that -regulate the appearance of volcanic phenomena. The study of the laws -of volcanic action is yet in its infancy. But the establishment of -observatories on Vesuvius and Etna 18 fall of promise for the future, -and when we consider the advances which have been made, during the last -one hundred years, in our knowledge of the true nature of volcanic -action, we need not despair that the extension of the same methods -of inquiry will lead to equally important results concerning the -conditions which determine and the laws which govern it. - -In the meanwhile, it is no small gain to have established the fact that -volcanic phenomena, divested of all those wonderful attributes with -which superstition and the love of the marvellous have surrounded them, -are operations of nature obeying definite laws, which laws we may hope -by careful observation and accurate reasoning to determine; and that -the varied appearances, presented alike in the grandest and feeblest -outbursts, can all be referred to one simple cause--namely, the escape, -from the midst of masses of molten materials, of imprisoned steam or -water-gas. - - - - -CHAPTER III. - -THE PRODUCTS OF VOLCANIC ACTION. - - -While Spallanzani was engaged in investigating the nature of the action -going on at Stromboli and other Italian volcanoes, his contemporary -Dolomieu was laying the foundation of another important branch of -vulcanology by studying the characters of the different materials of -which volcanoes are built up. Since the publication of Dolomieu's -admirable works on the rocks of the Lipari and Ponza Islands, science -has advanced with prodigious strides. The chemist has taught us how -to split up a rock into its constituent elements and to determine the -proportions of these to one another with mathematical precision; the -mineralogist has done much in the investigation of the characters and -mode of origin of the crystalline minerals which occur in these rocks; -and the microscopist has shown how the minute internal structure of -these rocks may be made clearly manifest. We shall proceed to give a -sketch of the present state of knowledge obtained by these different -kinds of investigations, concerning the materials which are ejected -from volcanic vents. - -The most abundant of the substances which are ejected from volcanoes -is steam or water-gas, which, as we have seen, issues in prodigious -quantities during every eruption. But with the steam a great number -of other volatile materials frequently make their appearance. The -chief among these are the add gases known as hydrochloric acid, -sulphurous acid, sulphuretted hydrogen, carbonic add, and boracic acid; -and with these acid gases there issue hydrogen, nitrogen, ammonia, -the volatile metals arsenic, antimony, and mercury, and some other -substances. In considering the nature of the products which issue from -volcanic fissures, it must be remembered that many substances which -under ordinary circumstances do not exhibit marked volatility are -nevertheless easily carried away in fine particles when a current of -steam is passed over them. As we shall have to point out in the sequel, -different volatile substances have a tendency to make their appearance -at volcanic vents according as the intensity of the action going on -within it varies. - -The volatile substances issuing from volcanic fissures at high -temperatures react upon one another, and many new compounds are thus -formed. We have already seen how, by the action of sulphurous acid -and sulphuretted hydrogen on each other, the sulphur so common in -volcanic districts has been separated and deposited. The hydrochloric -acid acts very energetically on the rocks around the vents, uniting -with the iron in them to form the yellow ferric-chloride. The rocks -all round a volcanic vent are not unfrequently found coated with this -yellow substance, which is almost always mistaken by casual observers -for sulphur. In many volcanoes the constant passage through the rocks -of the various acid gases has caused nearly the whole of the iron, -lime, and alkaline materials of the rocks to be converted into soluble -compounds known as sulphates, chlorides, carbonates, and borates; and, -on the removal of these by the rain, there remains a white, powdery -substance, resembling chalk in outward appearance, but composed of -almost pure silica. There are certain cases in which travellers have -visited volcanic islands where chemical action of this kind has gone on -to such an extent, that they have been led to describe the islands as -composed entirely of chalk. - -[Sidenote: GASES EMITTED FROM VOLCANOES.] - -Some of the substances issuing from volcanic vents, such as hydrogen -and sulphuretted hydrogen, are inflammable, and when they issue at a -high temperature, these gases burst into flame the moment that they -come into contact with the air. Hence, when volcanic fissures axe -watched at night, faint lambent flames are frequently seen playing over -them, and sometimes these flames are brilliantly coloured, through the -presence of small quantities of certain metallic oxides. Such volcanic -flames, however, are scarcely ever strongly luminous and, as we have -already seen, the red, glowing light which is observed over volcanic -mountains in eruption is due to quite another cause. The study by the -aid of the spectroscope of the flames which issue from volcanic vents -promises to throw much new light on the rarer materials ejected by -volcanoes. Spectroscopic observations of this kind have already been -commenced by Janssen, at Stromboli and Santorin. - -Some of the volatile substances issuing from volcanic vents, are at -once deposited when they come in contact with the cool atmosphere, -others form new compounds with one another and the constituents of the -atmosphere, while others again attack the materials of the surrounding -rocks and form fresh chemical compounds with some of their ingredients. -Thus, there are continually accumulating on the sides and lips of -volcanic fissures deposits of sulphates, chlorides, and borates of -the alkalies and alkaline earths, with sal-ammoniac, sulphur, and the -oxides and sulphides of certain metals. The lips of the fissures from -which steam and acid gases issue in volcanoes are constantly seen to -be coated with yellow and reddish-brown incrustations, consisting of -mixtures, in varying proportions, of these different materials, and -these sometimes assume the form of stalactites and pendent masses. - -[Sidenote: DEPOSITS AROUND VOLCANIC VENTS.] - -Some of these products of volcanic action are of considerable -commercial value. At Vulcano regular chemical works have been -established in the crater of the volcano, by an enterprising Scotch -firm, a great number of workmen being engaged in collecting the -materials which are deposited around the fissures, and are renewed -by the volcanic action almost as soon as they are removed. In fig. -6, I have given a sketch of this singular spot, taken from the high -ground of the neighbouring Island of Lipari. From the village at -the foot of the volcano, where the workmen live, a zig-zag road has -been constructed leading up the side, and down into the crater of -the volcano. On this road, workmen and mules, laden with the various -volcanic materials, may be seen constantly passing up and down. - -[Illustration: Fig. 6.--View of Vulcano, with Vulcanello in the -foreground taken from the south end of the Island of Lipari.] - -Vulcano appears to have been frequently in a state of violent eruption -during the past 2,000 years--the last great outburst having taken place -in 1786. In 1873 the activity in the crater of Vulcano suddenly became -more pronounced in character, and the workmen hastened to escape from -the dangerous spot, but, before they could do so, several of them -were severely injured by the explosions. After this outburst, which -did not prove to be of very violent character, the quantity of gases -issuing from the fissures in the crater was for a time much greater -than before, and the productiveness of these great natural chemical -works was proportionately increased: but eventually the action died out -almost entirely. The chief products of Vulcano which are of commercial -value, are sal-ammoniac, sulphur, and boracic acid. At one time it was -even contemplated that great leaden chambers should be erected over the -principal fissures at the bottom of the crater of Vulcano, in which -chambers the volatile materials might be condensed and collected. The -change in the condition of the volcano has unfortunately prevented the -carrying out of this bold project. - -Besides the volatile substances which issue from volcanic vents, -mingling with the atmosphere or condensing upon their sides, there are -also many solid materials ejected, and these may accumulate around the -orifices, till they build up mountains of vast dimensions, like Etna, -Teneriffe, and Chimborazo. Some of these solid materials are evidently -fragments of the rock-masses, through which the volcanic fissure has -been rent; these fragments have been carried upwards by the force of -the steam-blast and scattered over the sides of the volcano. But the -principal portion of the solid materials ejected from volcanic orifices -consists of matter which has been extruded from sources far beneath the -surface, in a highly-heated and fluid or semi-fluid condition. - -[Sidenote: EJECTED ROCK-FRAGMENTS.] - -The fragments torn from the sides of volcanic fissures consist of the -rocks through which the eruptive forces may happen to have opened their -way; pieces of sandstone, limestone, slate, granite, &c., are thus -frequently found in considerable numbers among materials which build up -volcanic mountains. Thus, some of the volcanic cones in the Eifel are -very largely made up of fragments of slate, which have been torn from -the sides of the vents by the uprushing currents of steam. At Vesuvius -masses of limestone are frequently ejected, and may be picked up all -over the slopes of the mountains. These limestone-fragments frequently -contain fossils, and Professor Guiscardi, of Naples, has been able to -collect several hundred species of shells, transported thus by volcanic -action from the rock-masses which form the foundation of the volcano -of Vesuvius. The action of water at a high temperature, and under -such enormous pressure as must exist beneath volcanic mountains, has -often produced changes in the rocks of which fragments are ejected -from volcanic vents. The so-called 'lava' ornaments, which are so -extensively sold at Naples, are not made from the materials to which -geologists apply that name, but from the fragments of altered limestone -that have been torn from the rocks beneath the mountain, and scattered -by the eruptive forces all over its sides. The chemical action of the -superheated and highly-compressed steam on the rocks beneath volcanoes -frequently results in the formation of beautifully crystallised -minerals. Such crystallised minerals abound in the rock-fragments -scattered over the sides of Vesuvius and other volcanoes, both active -and extinct. They have been formed in the great chemical laboratories -which exist beneath the volcano, and have been brought to the surface -by the action of the steam-jets issuing from its fissures. - -Of still greater interest are those materials which issue from volcanic -orifices in an incandescent, and often in a molten, condition, and -which are evidently derived from sources far below the earth's surface. -It is to these materials that the name of 'lavas' is properly applied. - -Lavas present a general resemblance to the slags and clinkers which -are formed in our furnaces and brick-kilns, and consist, like them, of -various stony substances which have been more or less perfectly fused. -When we come to study the chemical composition and the microscopical -structure of lavas, however, we shall find that there are many respects -in which they differ entirely from these artificial products. - -Let us first consider the facts which are taught us concerning the -nature and origin of lavas, by a chemical analysis of them. - -[Sidenote: CHEMICAL COMPOSITION OF LAVAS.] - -Of the sixty-five or seventy chemical elements, only a very small -number occur at all commonly in lavas. Eight elements, indeed, make -up the great mass of all lavas--these are oxygen, silicon, aluminium, -magnesium, calcium, iron, sodium, and potassium. But even these -eight elements are present in very unequal proportions. Oxygen makes -up nearly one-half the weight of all lavas. Almost all the other -elements found in lavas exist in combination with oxygen, so that -lavas consist entirely of what chemists call 'oxides.' This is a most -remarkable circumstance, which, as we shall presently see, is of great -significance. The metalloid silicon makes up about one-fourth of the -weight of most lavas, and the metal aluminium about one-tenth. The -other five elements vary greatly in their relative proportions in -different lavas. - -In all lavas the substance which forms the greatest part of the mass -is the compound of oxygen and silicon, known as silica or silicic -acid. In its pure form, this substance is familiar to us as quartz, or -rock-crystal and flint. Silica is present in all lavas in proportions -which vary from one-half to four-fifths of the whole mass. Now, this -substance, silica, has the property of forming more complex compounds -by uniting with the other oxides present in lavas--namely, the oxides -of aluminium, magnesium, calcium, iron, potassium, and sodium. Silica -is called by chemists an _acid_, the other oxides in lavas are termed -_bases_, and the compounds of silica with the bases are known as -_silicates_. Hence we see that lavas are composed of a number of -different silicates--the silicates of aluminium, magnesium, calcium, -iron, potassium, and sodium. - -The above statements will perhaps be made clearer by the accompanying -table from which it will be seen that lavas are compounds in varying -proportions of six kinds of salts--namely, the silicates of alumina, -magnesia, lime, iron, potash, and soda. - -Composition of Lavas. - - Elements Binary Compounds Salts - Acid Bases - { Silicon Silica--+ - { | - O { Aluminum +--Alumina Silicate of Alumina - x { | - y { Magnesium +--Magnesia " " Magnesia - g { | - e { Calcium +--Lime " " Lime - n { | - { Iron +--Iron " " Iron - { | - { Potassium +--Potash " " Potash - { | - { Sodium +--Soda " " Soda - - -Now, in some lavas the acid constituent, or silica, is present in much -larger proportions than in others. Those lavas with a large proportion -of silica are called 'acid lavas,' those with a lower percentage of -silica, and therefore a higher proportion of the bases, are known as -the 'basic lavas.' It is convenient to employ the term 'intermediate -lavas' for those in which the proportion of silica is lower than in the -acid lavas, and the proportion of the bases is lower than in the basic -lavas. - -The acid lavas contain from 66 to 80 per cent, of silica; they are poor -in lime, magnesia, and oxide of iron, but rich in potash and soda. The -basic lavas contain from 45 to 55 per cent, of silica; they are rich in -magnesia, lime, and oxide of iron, but poor in soda and potash. In the -intermediate lavas the proportion of silica varies from 55 to 66 per -cent. - -As the basic-lavas contain a larger proportion of oxide of iron and -other heavy oxides than the acid-lavas, the former have usually a -higher specific gravity than the latter; it is, indeed, possible in -most cases to distinguish between these different varieties by simply -weighing them in water and in air. - -[Sidenote: DIFFERENT KINDS OF LAVA.] - -The basic lavas are usually of much darker colour than the add -lavas--the terms acid lavas, intermediate lavas, and basic lavas -correspond indeed pretty closely with the names trachytes, greystones -and basalt, which were given to the varieties of lavas by the older -writers on volcanoes, at a time when their chemical constitution had -not been accurately studied. Fresh lavas of acid composition are -usually nearly white in colour, intermediate lavas are of various -tints of grey, and basic lavas nearly black. It must be remembered, -however, that colour is one of the least persistent, and therefore -one of the least valuable, characters by means of which rocks can -be discriminated, and also that by exposure to the influence of the -atmospheric moisture the iron present in all lavas is affected, and -the lavas belonging to all classes, when weathered, assume reddish and -reddish-brown tints. - -Geologists have devised a great number of names for the various kinds -of lava which have been found occurring round volcanic vents in -different parts of the world, and the study of these varieties is full -of interest. For our present purpose, however, it will be sufficient -to state that they nearly all fall into five great groups, known as -the Rhyolites, the Trachytes, the Andesites, the Phonolites, and the -Basalts. The Rhyolites are acid lavas, the Basalts are basic lavas, -and the Trachytes, Andesites, and Phonolites, different kinds of -intermediate lavas, distinguished by the particular minerals which they -contain. - -Before we part from this subject of the classification of lavas -according to their chemical composition, it will be well to point -out that there exists a small group of lavas which stand quite by -themselves, and cannot be referred to either of the classes we have -indicated. They contain a smaller proportion of silica, and a much -larger proportion of magnesia and oxide of iron than the other lavas, -and may be made to constitute a small sub-group, to which we may apply -the term of 'ultra-basic lavas.' Although much less widely distributed -than the other varieties, they are, in some respects, as we shall -presently have to point out, of far greater interest to the geologist -than all the other kinds of lavas. - -[Sidenote: MINUTE STRUCTURE OF LAVAS.] - -We will now proceed to consider the facts which are brought to light -concerning the nature of lavas, when they are studied by the aid -of the microscope. Although most lavas appear at first sight to be -opaque substances, yet it is easy to prepare slices of them which are -sufficiently thin to transmit light. In such thin transparent slices -we are able to make out, by the aid of the microscope, certain very -interesting details of structure, which afford new and important -evidence bearing on the mode of origin of these rocks. - -Host lavas are capable of being melted by the heat of our furnaces; -but the different kinds of lava vary greatly in the degree of their -fusibility. The basic lavas, or those with the smallest proportion of -silica, are usually much more easily fusible than those which contain a -high percentage of silica, the add lavas. - -Now, it is a very noteworthy circumstance, that when a lava is -artificially fused it assumes on cooling very different physical -characters to those which were presented by the original rock. - -If we examine the freshly-broken surface of a piece of lava, we -shall, in most cases, find that it contains a great number of those -regular-shaped bodies which we call crystals; in some cases these -crystals are so small as to be scarcely visible to the naked eye, in -others they may be an inch or more in length. Most lavas are thus seen -to be largely made up of crystals of different minerals. The minerals -which are usually contained in lavas are quartz, the various kinds of -felspar, augite, hornblende, the different kinds of mica, olivine, and -magnetite. - -But when a piece of lava is melted in a furnace, all these crystalline -minerals disappear, and the resulting product is the homogeneous -substance which we call glass. If, as many suppose, lavas acquire the -fluidity which they possess when issuing from volcanic vents as the -result of simple fusion it is strange that artificially fused lavas do -not agree more closely in character with the natural products. - -A careful examination of different kinds of lavas, however, will -show that they vary very greatly in character among themselves. Some -lavas are as perfectly glassy in structure as those which have been -artificially fused, while others contain great numbers of crystals, -which may sometimes be of very large size. - -If we prepare thin transparent slices of these different kinds of -lavas, and examine them by the aid of the microscope, we shall find -that lavas are made up of two kinds of materials, a base or groundmass -of a glassy character, and distinct crystals of different minerals, -which are irregularly distributed through this glassy base, like the -raisins, currants, and pieces of candied peel in a cake. In some -cases the glassy base makes up the whole mass of the rock; in others, -smaller or larger numbers of crystals are seen to be scattered through -a glassy base; while in others again the crystals are so numerous that -the presence of an intervening glassy base or groundmass can only be -detected by the aid of the microscope. - -[Sidenote: STUDY OF LAVAS WITH THE MICROSCOPE.] - -If thin slices of the glassy materials of lavas be examined with high -magnifying powers, new and interesting facts are revealed. Through -the midst of the clear glassy substance cloudy patches are seen to be -diffused; and, if we examine them with a still higher power, these -cloudy patches resolve themselves into innumerable particles, some -transparent and others opaque, having very definite outlines. At the -same time fresh cloudy patches are brought into view, which can only -be resolved by yet higher powers of the microscope. In examining these -natural glasses by the aid of the microscope, we are forcibly reminded -of what occurs when the 'Milky Way' and some other parts of the heavens -are studied with a telescope. As the power of the instrument is -increased the nebulous patches are resolved into distinct stars, but -fresh nebulous masses come into view, which are in turn resolved into -stars, when higher powers of the instrument are employed. - -In the Frontispiece, No. 1 illustrates the appearance presented by -these volcanic glasses when examined with a high power of a microscope. -Through a glassy base is seen a number of diffused nebulous patches, -which are in places resolved into definite particles. - -These minute particles of definite form, which the microscope has -revealed in the midst of the glassy portions of lava, have received the -name of microliths, or crystallites. The study of the characters and -mode of arrangement of these microliths or crystallites has in recent -years thrown much new light on the interesting problems presented by -lavas. - -In some glassy lavas the microliths or crystallites, instead of being -indiscriminately diffused through the mass of the base or groundmass, -are found to be collected together into groups of very definite form. -In No. 2 of the Frontispiece we have a section of a glassy rock in -which the crystallites have united together, so as to build up groups -presenting the most striking resemblance to fronds of ferns. Around -these groups spaces of dear glass have been left by the gathering up -of the crystallites, which in other parts of the mass are seen to be -equally diffused through it. In this formation of groups of microliths -we cannot but recognise the action of those crystalline forces, which -on frosty mornings cover our windows with a mimic vegetation composed -of icy particles. - -In other cases, again, the crystallites scattered through the glassy -portions of lavas unite in radial groups about certain centres, and -thus build up globular masses to which the name of 'sphærulites' has -been given. No. 3 in the Frontispiece illustrates the formation of -these sphærulites. - -Now, a careful study of the microliths or crystallites has proved that -they are the minute elements of which those wonderfully beautiful -objects which we call crystals are built up. In some cases we can -see that the crystallites are becoming united together in positions -determined by mathematical laws, and the group is gradually assuming -the outward form and internal structure of a crystal. In other cases -crystals may be found which are undergoing a disintegrating action, -and are then seen to be made up of minute elements similar to the -crystallites or microliths of glassy rocks. - -[Sidenote: CRYSTALLITES AND CRYSTALS.] - -The conclusion is confirmed by the fact that if we take an artificially -fused lava and allow it to cool slowly, it will be found that the -glassy mass into which it has resolved itself contains numerous -crystallites. If the cooling process be still further prolonged, these -crystallites will be found to have united themselves into definite -groups, and sometimes distinct crystals are formed in the mass; under -these circumstances the rock frequently loses its glassy appearance and -assumes a stony character. - -In connection with this subject, it may be mentioned that some years -ago a very ingenious invention was submitted to trial in the Works -of the Messrs. Chance, of Birmingham. It had been suggested that if -certain lavas of easy fusibility were melted and poured into moulds, -we might thus obtain elaborately ornamented stone-work, composed of -the hardest material, without the labour of the mason. The molten rock -when quickly cooled was found to assume the form of a black glass, but -when very slowly cooled passed into a stony material. Unfortunately, -it was found that this material did not withstand the weather like -ordinary building stones, and, in consequence, the manufacture had to -be abandoned. - -Now, the study of the products of volcanoes has led geologists to -recognise the true relations between glassy and crystalline rocks. - -In the amorphous mixture of various silicates which compose a glass, -chemical affinity causes the separation of certain portions of -definite composition, and these form the microliths or elements of -which different crystalline minerals are built up. Under the influence -of the crystalline forces, there is a great shaking or agitation in -the mass, and the microliths of similar kind come together and become -united, like the fragments in Ezekiel's valley of dry bones. - -Although we cannot see this process taking place under our eyes, -in a mass of lava, yet we may study specimens in which the action -has been arrested in its different stages. In order to understand -the development of an acorn into an oak-tree, it is not necessary -to watch the whole series of changes in a particular case. A visit -to an oak-thicket, in which illustrations of every stage of the -transformation may be found, will afford us equally certain information -on the subject. - -In the same way by the examination of such a series of rock-sections -as that represented in the Frontispiece, we may understand how, in the -midst of a mass of mixed silicates constituting a natural glass, the -separation of microliths takes place; these unite into groups which are -the skeletons of crystals, and finally, by the filling up of the empty -spaces in these skeletons, complete crystals are built up. The series -of operations may, however, be interrupted at any stage, and this stage -we may have the chance of studying. - -[Sidenote: GLASSY AND CRYSTALLINE LAVAS.] - -We are able, as we shall show in a future chapter, to examine many -rock-masses that have evidently formed the reservoirs from which -volcanoes have been supplied, and others that fill up the ducts which -constituted the means of communication between these subterranean -reservoirs, and the surface of the earth. Now in these subterranean -regions the lavas have been placed under conditions especially -favourable for the action of the crystalline forces--they must have -cooled with extreme slowness, and they must have been under an enormous -pressure, produced in part by the weight of the superincumbent rocks, -and in part by the expansive force of the imprisoned steam. We are not, -therefore, surprised to find that in these subterranean regions, the -lavas, while retaining the same chemical composition, have assumed a -much more perfectly crystalline condition. In some cases, indeed, the -whole rock has become a mass of crystals without any base or groundmass -at all. - -An examination of the Frontispiece will illustrate this perfect -gradation from the glassy to the crystalline condition of lavas. No. -1 represents a glass through which microliths or crystallites of -different dimensions and character are diffused. In Nos. 2 and 3, -these crystallites have united to form regular groups. In No. 4, which -may be taken as typical of the features presented by most lavas, we -have a glassy groundmass containing microliths (a 'crypto-crystalline -base'), through which distinct crystals are distributed. Nos. 5 and 6 -illustrate the characters presented by lavas which have consolidated -at considerable depths beneath the surface; in the former we have -a mans of small crystals (a 'micro-crystalline base') with larger -crystals scattered through it; while the latter is entirely made up of -large crystals without any trace of a base or groundmass. - -Now, as all lavas are found sometimes assuming the glassy condition at -the surface, so when seen in the masses which have consolidated with -extreme slowness, and under great pressure, in subterranean regions, -the same materials are found in the condition of a rock which is built -up entirely of crystals. Chemists have found that artificial mixtures -of silicates in which soda and potash are present in considerable -quantities, have a great tendency to assume the glassy condition on -cooling from a state of fusion, and glass manufacturers are always -careful to use considerable proportions of the alkalis as ingredients, -in making glass. It is found, in like manner, that those lavas which -contain the largest portion of the silicates of soda and potash (the -'acid lavas') most frequently assume the condition of a natural glass. - -Geologists have given distinct names to the glassy and the perfectly -crystalline conditions of the different kinds of lavas, the glassy -varieties being found in masses which have cooled rapidly near the -surface, and the crystalline varieties in masses which have cooled -slowly at great depths. The names of these two conditions of the five -great classes into which we have divided lavas are as follows:-- - -[Sidenote: HIGHLY CRYSTALLINE IGNEOUS ROCKS.] - - - _Crystalline Forms._ _Lavas._ _Glassy Forms._ - - Granite Rhyolite } - Syenite Trachyte } Obsidian. - Diorite Andesite } - Miascite Phonolite } - Gabbro Basalt Tachylyte. - -As vitreous rocks have little in their general appearance to -distinguish them from one another, the glassy forms of the first -four classes of lava have not hitherto received distinct names, but -have been confounded together under the name of obsidian. If we -determine the specific gravities of rocks having the same composition -but different structures, we shall find that they become heavier in -proportion as the crystalline structure is developed in them. Thus -gabbro is heavier, but tachylyte is lighter than basalt, bulk for bulk, -though all have the same chemical composition. - -Nor are the crystals contained in lavas less worthy of careful study, -by the aid of the microscope, than the more or less glassy groundmass -in which they are embedded. Mr. Sorby has shown that the crystals found -in lavas, exhibit many interesting points of difference from those -which separate out in the midst of a mass of the same rock, when it has -been artificially melted and slowly cooled. There are other facts which -also point to the conclusion that, while the glassy groundmass of lavas -may have been formed by cooling from a state of fusion, the larger and -well-formed crystals in these lavas must have been formed under other -and very different conditions. - -The larger crystals in lavas exhibit evidence of having been slowly -built up in the midst of a glassy mass, containing crystallites and -small crystals. We can frequently detect evidence of the interruptions -which have occurred in the growth of these crystals in the concentric -zones of different colour or texture which they exhibit; and portions -of the glassy base or groundmass are often found to have been caught up -and enclosed in these crystals during their growth. - -But when we find, as in the porphyritic pitchstones, a glassy base -containing only minute crystallites, through which large and perfectly -formed crystals are distributed, we can scarcely doubt that the minute -crystallites and the larger crystals have separated from the base under -very different conditions. This is indicated by the bet that we detect -in these cases no connecting links between the embryo microliths and -the perfect crystals; and a confirmation of the conclusion is seen in -the circumstance that many of the crystals are found to have suffered -injury as if from transport, their edges and angles being rounded and -abraded, and portions being occasionally broken off from them. - -Hence we are led to conclude that the larger crystals in lavas were -probably separated from the amorphous mass in the subterranean -reservoirs beneath the volcano, and were carried up to the surface in -the midst of the liquefied glassy material which forms the groundmass -of lavas. When we come to examine these crystals more closely, we find -that certain very curious phenomena are exhibited by them which lend -powerful support to this conclusion. - -[Illustration: Fig. 7.--Minute Cavities, containing Liquids, in the -Crystals of Rocks.] - -[Sidenote: LIQUID CAVITIES IN CRYSTALS.] - -It is found convenient by geologists to designate those rocks which -have consolidated in deep-seated portions of the earth's crust -as Platonic Rocks, confining the name of Volcanic rocks to those -consolidating At the surface; but Plutonic and Volcanic Rocks shade -into one another by the most insensible gradations. - -When the crystals embedded in granitic rocks, and in some lavas, are -examined with the higher powers of the microscope, they are frequently -seen to contain great numbers of excessively minute cavities. Each of -these cavities resembles a small spirit-level, having a quantity of -liquid and a bubble of gas within it. In fig. 7 we have given a series -of drawings of these cavities in crystals as seen under a high power -of the microscope. In No. 1 a group of such cavities is represented, -one of which is full of liquid, while two others are quite empty; the -remaining cavities all contain a liquid with a moving bubble of gas. -In No. 2 two larger cavities are shown, containing a liquid and a -bubble of gas; and it will be seen from these how varied in form these -cavities sometimes are. In Nos. 3, 4 and 6 the liquid in the cavities -contains, besides the bubbles, several, minute crystals; and in No. 6 -we have a cavity containing two liquids and a bubble. - -In the largest of such cavities the bubble is seen to change its -place so as always to lie at the upper side of the cavity, when the -position of the latter is altered, just as in a spirit-level. But in -the smallest cavities the bubbles appear to be endowed with a power of -spontaneous movement; like imprisoned creatures trying to escape, these -bubbles are seen continually oscillating from side to side and from end -to end of the cavities which enclose them. In fig. 8 a minute cavity -containing a liquid and bubble is shown, the path pursued by the latter -in its wonderful gyrations being indicated by the dark line. These -cavities are exceedingly minute, and so numerous that in some crystals -there must be millions of them present; indeed, in certain cases, as -we increase the magnifying power of our microscopes, new and smaller -cavities continually become visible. It has been estimated that in some -instances the number of these minute liquid-cavities in the crystals of -rocks amounts to from one thousand millions to ten thousand millions in -a cubic inch of space. - -[Illustration: Fig. 8--Minute Liquid-cavity in a Crystal, with a moving -Bubble. (The path of the bubble is indicated by the dark line.)] - -[Sidenote: NATURE OF LIQUIDS IN CAVITIES.] - -What is the nature of the liquids which are thus imprisoned in these -cavities contained in the crystals of lavas and granites? Careful -experiments have given a conclusive answer to this question. In many -cases the liquid is water, usually containing considerable quantities -of saline matter dissolved in it. Sometimes the saline matters are -present in such abundance that they cannot all pass into solution, but -crystallise out, as in fig. 7--Nos. 3, 4, 5--where cubic crystals of -the chlorides of sodium and potassium are seen floating in the liquid; -in other cases the liquid is a hydrocarbon like the mineral oil which -is present in great abundance in deep-seated rocks in many parts of the -globe. But in some other cases the liquid contained in the cavities -of crystals is found to be one which could scarcely be anticipated -to occur under such circumstances--the gas known as carbonic add, -which under extreme pressure can be reduced to a liquid condition. In -cavities containing liquefied carbonic acid, if the rock be warmed up -to 86° or 90° Fahrenheit the bubble suddenly vanishes, sometimes with -an appearance like ebullition or boiling, as represented in fig. 9. -Now the temperature which we have indicated is the 'critical point' of -carbonic acid, and above that temperature it cannot exist in a liquid -condition, however great may be the pressure to which it is subjected. -The liquid has been converted into a gas which completely fills the -cavity. The carbonic acid in the cavities of crystals has frequently -been isolated and its nature placed beyond doubt by spectroscopic and -ordinary chemical tests. - -The presence of these liquids in the cavities of crystals clearly -proves that the latter must have been formed under enormous pressure--a -pressure sufficiently great to reduce, not only steam, but also -volatile hydrocarbons and even gaseous carbonic acid, to the bulk of a -liquid. - -[Illustration: Fig. 9.--Cavity in Crystal containing Carbonic-Acid Gas -at a temperature of 86° F., and passing from the liquid to the gaseous -condition.] - -Such conditions of enormous pressure we may infer to exist in the -deep-seated reservoirs beneath volcanoes, where, besides the weight -of the superincumbent rock-masses, we have the compressing force of -great quantities of elastic vapour held in confinement. The crystals of -which granitic rocks are entirely built up exhibit clear evidence of -having been all formed under these conditions of enormous pressure. The -glassy base or groundmass of lavas, on the other hand, presents all the -characters of materials that have cooled from a state of fusion. Most -lavas consist in part of crystals, exhibiting fluid-cavities like those -present in granite, and in part of a base, which has evidently been -formed by the cooling of a fused mass. We are therefore justified in -concluding that the crystals have been formed in subterranean recesses, -and that the groundmass or base has consolidated at the surface. The -bearing of these conclusions upon some of the great problems presented -by volcanoes we shall have occasion to point out in the sequel. - -[Sidenote: CAUSE OF MOVEMENT OF BUBBLES.] - -One of the most interesting inquiries suggested by the study of -the liquid-cavities in volcanic rocks is that of the cause of the -apparently spontaneous movement of the bubbles which we have described -as taking place in some of the smaller of them. The ingenious -experiments of Mr. Noel Hartley have suggested to Professor Stokes -an explanation which is probably the true one. It appears that these -minute globes of vapour are in such a state of unstable equilibrium as -to be affected by the smallest changes of temperature, and that the -variations in the heat of the atmosphere, due to currents of air and -the movement of warm or cold bodies through it, are sufficient to cause -the oscillation of these sensitively poised bubbles. - -The short account which we have been able to give in the foregoing -pages of the researches that have been carried on concerning the nature -of the materials ejected from volcanoes will serve to show that these -investigations have already made known many facts of great interest, -and that the farther pursuit of them is full of the highest promise. -To the scientific worker no subject is too vast for his research, no -object so minute as to be unworthy of his most patient study. In some -of our future inquiries concerning the nature of volcanic action, we -shall be led to an investigation of the phenomena displayed in the sun, -moon, comets and other great bodies of the universe; but another road -to truths of the same grandeur and importance is found, as we have -seen, in an examination of the mode of development of crystallites, and -a study of the materials contained in the microscopic cavities of the -minutest crystals. - - - - -CHAPTER IV. - -THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS. - - -The escape of great quantities of steam and other gases from the midst -of a mass of fluid or semi-fluid lava gives rise to the formation of -vast quantities of froth or foam upon its surface. This froth or foam, -which is formed upon the surface of lava by the escape of gaseous -matters from within it, is made up of portions of the lava distended -into vesicles, in the same way that bubbles are formed on the surface -of water. It bears precisely the same relation to the liquid mass of -lava that the white crest of foam upon an advancing wave does to the -sea-water, from the bubbles of which it is formed. - -This froth upon the surface of lavas varies greatly in character -according to the nature of the material from which it is formed. In -the majority of cases the lavas consist, as we have seen, of a mass -of crystals floating in a liquid magma, and the distension of such a -mass by the escape of steam from its midst gives rise to the formation -of the rough cindery-looking material to which the name of 'scoria' -is applied. But when the lava contains no ready-formed crystals, but -consists entirely of a glassy substance in a more or less perfect state -of fusion, the liberation of steam gives rise to the formation of the -beautiful material known as 'pumice.' Pumice consists of a mass of -minute glass bubbles; these bubbles have not usually, however, retained -their globular form, but have been elongated in one direction through -the movement of the mass while it was still in a plastic state. - -The steam frequently escapes from lava with such violence that the -froth or scum on its surface is broken up and scattered in all -directions, as the foam crests of waves are dispersed by the wind -during a storm. In this way fragments of scoria or pumice are often -thrown to the height of many hundreds or thousands of feet into the -atmosphere, as we have seen is the case at Stromboli and Vesuvius. -Indeed, during violent eruptions, a continuous upward discharge of -these fragments is maintained, the ragged cindery masses hurtling one -another in the atmosphere, as they are shot perpendicularly upwards -to an enormous height and fall back into the vent; or they may rise -obliquely and describe curves so as to descend outside the orifice from -which they were ejected. - -[Sidenote: FINENESS OF VOLCANIC DUST.] - -During their upward discharge and downward fall, the cindery fragments -are by attrition continually reduced to smaller dimensions. The noise -made by these fragments, as they strike against one another in -the air during their rise and fall, is one of the most noteworthy -accompaniments of volcanic eruptions. It has been noticed that in many -cases there is a constant diminution in the size of the fragments -ejected during a volcanic outburst, this being doubtless due to the -friction of the masses as they are ejected and re-ejected from the -vent. Thus it is related by Mr. Poulett Scrope, who watched the -Vesuvian eruption of 1822, which lasted for nearly a month, that -during the earlier stages of the outburst fragments of enormous size -were thrown out of the crater, but by constant re-ejection these were -gradually reduced in size, till at last only the most impalpable dust -issued from the vent. This dust filled the atmosphere, producing in -the city of Naples 'a darkness that might be felt,' and so excessively -finely divided was it, that it penetrated into all drawers, boxes, and -the most closely fastened receptacles, filling them completely. Mr. -Whymper relates that, while standing on the summit of Chimborazo, he -witnessed an eruption of Cotopaxi, which is distant more than fifty -miles from the former mountain. The fine volcanic dust fell in great -quantities around him, and he estimated that no less than two millions -of tons must have been ejected during this slight outburst. Professor -Bonney has examined this volcanic dust from Cotopaxi, and calculates -that it would take from 4,000 to 25,000 particles to make up a grain in -weight. - -Various names have been given by geologists to the fragments ejected -from volcanic vents, which, as we have seen, differ greatly in their -dimensions and other characters. Sometimes masses of more or less -fluid lava are flung bodily to a great height in the atmosphere. -During their rise and fall these masses are caused to rotate, and in -consequence assume a globular or spheroidal form. The water imprisoned -in these masses, during their passage through the atmosphere, tends to -expand into steam, and they become more or less completely distended -with bubbles. Such masses, which sometimes assume very regular and -striking forms, are known as 'volcanic bombs.' Many volcanic bombs have -a solid nucleus of refractory materials. The large, rough, angular, -cindery-looking fragments are termed 'scoriæ.' When reduced to the -dimensions of a marble or pea they are usually called by the Italian -name of 'lapilli.' The still finer materials are known as volcanic sand -and dust. - -There are, however, two names which are frequently applied to these -fragmentary materials ejected from volcanoes, which are perhaps -liable to give rise to misconception. These are the terms 'cinders' -and 'ashes.' It must be remembered that the scoriæ or cindery-looking -masses are not, like the cinders of our fires, the product of the -partial combustion of a material containing inflammable gases, but are, -like the clinkers of furnaces and brick-kilns, portions of partially -vitrified and fused rock distended by gases. So, too, volcanic ashes -only resemble the ashes of our grates in being very finely divided; -they are not, like the latter, the incombustible residue of a mass -which has been burnt. - -[Sidenote: VOLCANIC BOMBS AND PELE'S HAIR.] - -The glassy lavas, when distended by escaping gases, give rise to the -formation of pumice, the white colour of which, as in the case of the -foam of a wave, is due to the reflection of a portion of the light in -its frequent passage from one medium to another--in this case from -air to glass, and from glass to air. The volcanic bombs formed from -glassy lavas are often of especially beautiful and regular forms. -Sometimes the passage of steam through a mass of molten glass produces -large quantities of a material resembling spun glass. Small particles -or shots of the glass are carried into the air and leave behind them -thin, glassy filaments like a tail. At the volcano of Kilauea in Hawaii -this filamentous volcanic glass is abundantly produced, and is known -as 'Pele's Hair'--Pele being the name of the goddess of the mountain. -Birds' nests are sometimes found composed of this beautiful material. -In recent years an artificial substance similar to this Pele's hair -has been extensively manufactured by passing jets of steam through the -molten slag of iron-furnaces; it resembles cotton-wool, but is made up -of fine threads of glass, and is employed for the packing of boilers -and other purposes. - -The very finely-divided volcanic dust is often borne to enormous -distances from the volcano out of which it has been ejected. The force -of the steam-current carrying the fragments into the atmosphere is -often so great that they rise to the height of several miles above -the mountain. Here they may actually pass into the upper currents of -the atmosphere and be borne away to the distance of many hundreds -or thousands of miles. Hence it is not an unusual circumstance for -vessels at sea to encounter at great distances from land falling -showers of this finely divided, volcanic dust. We sometimes meet with -this far-travelled, volcanic dust under very unexpected circumstances. -Thus, in the spring of 1875 I had occasion to visit Prof. Vom Rath of -Bonn, who showed me a quantity of fine volcanic dust which had during -the past winter fallen in considerable quantities in certain parts -of Norway. This dust, upon microscopic examination, proved to be so -similar to what was known to be frequently ejected from the Icelandic -volcanoes that a strong presumption was raised that volcanic outbursts -had been going on in that island. On returning to England I found that -the first steamer of the season had just reached Leith from Iceland, -bringing the intelligence that very violent eruptions had taken place -during the preceding months. - -[Sidenote: DISPERSION OF PUMICE AND VOLCANIC DUST.] - -This finely-divided volcanic dust is thus carried by the winds and -spread over every part of the ocean. Everyone is familiar with the -fact that pumice floats upon water; this it does, not because it is a -material specifically lighter than water, but because cavities filled -with air make up a great part of its bulk. If we pulverise pumice, we -find the powder sinks readily in water, but the rock in its natural -condition floats for the same reason that an iron ship does--because -of the air-chambers which it encloses. When this pumice is ejected from -a volcano and falls into a river or the ocean, it floats for a long -time, till decomposition causes the breaking down of the thin glassy -partitions between the air chambers, and causes the admission of water -into the latter, by which means the whole mass gets water-logged. Near -the Liparis and other volcanic islands the sea is sometimes covered -with fragments of pumice to such an extent that it is difficult for a -boat to make progress through it, and the same substance is frequently -found floating in the open ocean and is cast up on every shore. - -During the year 1878 masses of floating pumice were reported as -existing in the vicinity of the Solomon Isles, and covering the surface -of the sea to such extent that it took ships three days to force their -way through them. Sometimes these masses of pumice accumulate in such -quantities along coasts that it is difficult to determine the position -of the shore within a mile or two, as we may land and walk about on -the great floating raft of pumice. Now, recent deep-sea soundings, -carried on in the 'Challenger' and other vessels, have shown that the -bottom of the deepest portion of the ocean, far away from the land, is -covered with these volcanic materials which have been carried through -the air or floated on the surface of the ocean. To these deeper parts -of the ocean no sediments carried down by the rivers are borne, and the -remains of calcareous organisms are, in these abysses, soon dissolved; -under such conditions, therefore, almost the only material accumulating -on the sea bottom is the ubiquitous wind- and wave-borne volcanic -products. These particles of volcanic dust and fragments of pumice by -their disintegration give rise to a clayey material, and the oxidation -of the magnetite, which all lavas contain, communicates to the mass a -reddish tint. This appears to be the true origin of those masses of -'red-clay' which, according to recent researches, are found to cover -all the deeper parts of the ocean, but which probably attain to no -great thickness. - -But while some portion of the materials ejected from volcanoes may -thus be carried by winds and waves, so as to be dispersed over every -part of the land and the ocean-bed, another, and in most cases by far -the largest, portion of these ejections falls around the volcanic vent -itself. It is by the constant accumulation of these ejected materials -that such great mountain masses as Etna, Teneriffe, Fusiyama, and -Chimborazo have been gradually built up around centres of volcanic -action. - -There are cases in which the formation of volcanic mountains on a small -scale has actually been observed by trustworthy witnesses. There are -other cases in which volcanic mountains of larger size can be shown -to have increased in height and bulk by the fall upon their sides and -summits of fragmentary materials ejected from the volcanic vent. In all -cases the examination of these mountain-masses leads to the conclusion -that they are entirely built up of just such materials as we constantly -see thrown out of volcanoes during eruption. - -[Sidenote: FORMATION OF VOLCANIC MOUNTAINS.] - -Thus we are led to the conclusion that all volcanic mountains are -nothing but heaps of materials ejected from fissures in the earth's -crust, the smaller ones having been formed during a single volcanic -outburst, the larger ones being the result of repeated eruptions from -the same orifice which may, in some cases, have continued in action for -tens or hundreds of thousands of years. - -No observer has done such useful work in connection with the study of -the mode of formation of volcanic mountains as our countryman, Sir -William Hamilton, who was ambassador at Naples from 1764 to 1800, and -made the best possible use of his opportunities for examining the -numerous volcanoes in Southern Italy. - -A little to the west of the town of Puzzuoli on the Bay of Naples there -stands a conical hill rising to the height of 440 feet above the level -of the Mediterranean, and covering an area more than half a mile in -diameter. Now we have the most conclusive evidence that in ancient -times no such hill existed on this site, which was partly occupied by -the Lucrine Lake, and the fact is recognised in the name which the hill -bears, that of Monte Nuovo, or the 'New Mountain.' See fig. 10. - -Sir William Hamilton rendered admirable service to science by -collecting all the contemporary records relating to this interesting -case, and he was able to prove, by the testimony of several -intelligent and trustworthy witnesses, that during the week following -the 29th of September, 1538, this hill had gradually been formed of -materials ejected from a volcanic vent which had opened upon this site. - -[Illustration: Fig. 10. Monte Nuovo (440 ft. high) on the shores of the -Bay of Naples.] - -[Sidenote: HISTORY OF THE FORMATION OF MONTE NUOVO.] - -The records collected by Hamilton with others which have been -discovered since his death prove most conclusively the following -facts. During more than two years, the country round was affected -by earthquakes, which gradually increased in intensity and attained -their climax in the month of September 1538; on the 27th and 28th of -that month these earthquake shocks are said to have been felt almost -continuously day and night. About 8 o'clock on the morning of the 29th, -a depression of the ground was noticed on the site of the future hill, -and from this depression, water, which was at first cold and afterwards -tepid, began to issue. Four hours afterwards the ground was seen to -swell up and open, forming a gaping fissure, within which incandescent -matter was visible. From this fissure numerous masses of stone, some of -them 'as large as an ox,' with vast quantities of pumice and mud, were -thrown: up to a great height, and these falling upon the sides of the -vent formed a great mound. This violent ejection of materials continued -for two days and nights, and on the third day a very considerable hill -was seen to have been built up by the falling fragments, and this hill -was climbed by some of the eye-witnesses of the eruption. The next day -the ejections were resumed, and many persons who had ventured on the -hill were injured, and several killed by the falling stones. The later -ejections were however of less violence than the earlier ones, and seem -to have died out on the seventh or eighth day after the beginning of -the outburst. The great mass of this considerable hill would appear, -according to the accounts which have been preserved, to have been built -up by the materials which were ejected during two days and nights. - -Monte Nuovo is a hill of truncated conical form, which rises to the -height of 440 feet above the waters of the Mediterranean, and is now -covered with thickets of stone-pine. The hill is entirely made up -of volcanic scoriæ, lapilli, and dust, and the sloping sides have -evidently been produced by these fragmentary materials sliding over one -another till they attained the angle of rest; just as happens with the -earth and stones tipped from railway-waggons during the construction of -an embankment. In the centre of this conical hill is a vast circular -depression, with steeply sloping sides, which is of such depth that its -bottom is but little above the sea-level. This cup-shaped depression is -the 'crater' of the volcano, and it has evidently been formed by the -explosive action which has thrown out the materials immediately above -the vent, and caused them to be accumulated around it. - -[Illustration: Fig. 11.--Map of the district around Naples, showing -Monte Nuovo and the surrounding volcanoes of older date.] - -The district lying to the west of Naples, in which the Monte Nuovo is -situated, contains a great number of hills, all of which present a most -striking similarity to that volcano. All these hills are truncated -cones, with larger or smaller circular depressions at their summits, -and they axe entirely composed of volcanic scoriæ, lapilli, and dust. -Some of these hills are of considerably larger dimensions than the -Monte Nuovo, while others are of smaller size, as shown in the annexed -map, fig. 11. No stranger visiting the district, without previous -information upon the subject, would ever suspect the fact that, while -all the other hills of the district have existed from time immemorial, -and are constantly mentioned in the works of Greek and Roman writers, -this particular hill of Monte Nuovo came into existence less than 350 -years ago. - -[Sidenote: OLDER VOLCANOES OF THE CAMPI PHLEGRÆI.] - -The evidently fused condition of the materials of which these hills are -built up is a dear sign of the volcanic action which has taken place -in it; and this feet was so fully recognised by the ancients that they -called the district the Campi Phlegræi, or 'the Burning Fields,' and -regarded one of the circular depressions in it as the entrance to Hades. - -It is impossible for anyone to examine this district without being -convinced that all the numerous cones and craters which cover it -have been formed by the same agency as that by which Monte Nuovo was -produced. We have shown that there is the most satisfactory historical -evidence as to what that agency was. - -Now volcanic cones with craters in their centres occur in great numbers -in many parts of the earth's surface. In some districts, like the -Auvergne, the Catacecaumene in Asia Minor, and certain parts of New -Zealand, these volcanic cones occur by hundreds and thousands. In some -instances, these volcanic cones have been formed in historic times, -but in the great majority of cases we can only infer their mode of -origin from their similarity to others of which the formation has been -witnessed. - -Most of the smaller volcanic hills, with their craters, have been -thrown up during a single eruption from a volcanic fissure; but, as -Hamilton conclusively proved, the grandest volcanic mountains must -have been produced by frequent repetitions of similar operations upon -the same site. For not only are these great volcanic piles found to be -entirely composed of materials which have evidently been ejected from -volcanic vents, but, when carefully watched, such mountains are found -undergoing continual changes in form, by the addition of materials -thrown out from the vent, and falling upon their sides. - -This fact will be well illustrated by a comparison of the series of -drawings of the summit of Vesuvius which were made by Sir William -Hamilton in 1767, and which we have copied in fig. 12. During the -earlier months of that year the summit of the mountain was seen to be -of truncated form, a great crater having been originated by the violent -outbursts of the preceding year. This condition of the mountain-top -is represented in the first figure of the series. The drawing made by -Hamilton, on July 8, shows that not only was the outer rim of the great -crater being modified in form by the fall of materials upon it, but -that in the centre of the crater a small cone was being gradually built -up by the quiet ejections which were taking place. - -[Illustration: Fig. 12.--Outlines of the Summit of Vesuvius during the -Eruption of 1767.] - -[Sidenote: CHANGES IN FORM OF VESUVIUS.] - -If we compare the drawings made at successive dates, we shall find that -the constant showers of falling materials were not only raising the -edge of the great crater but were at the same time increasing the size -of the small cone inside the crater. By the end of October the small -cone had grown to such an extent that its sides were confluent with -those of the principal cone, which had thus entirely lost its truncated -form and been raised to a much greater height. The comparison of these -drawings will be facilitated by the dotted lines, which represent -the outline of the top of the mountain at the preceding observation; -so that the space between the dotted and the continuous line in each -drawing shows the extent to which the bulk of the cone had increased in -the interval between two observations. - -But, although the general tendency of the action going on at volcanic -mountains is to increase their height and bulk by the materials falling -upon their summits and aides, it must be remembered that this action -does not take place by any means continuously and regularly. Not only -are there periods of rest in the activity of the volcano, during which -the rain and winds may accomplish a great deal in the way of crumbling -down the loose materials of which volcanic mountains are largely built -up, but sudden and violent eruptions may in a very short time undo the -slow work of years by blowing away the whole summit of the mountain at -once. Thus, before the great eruption of 1822, the cone of Vesuvius, by -the almost constant ejection of ashes during several years, had been -raised to the height of more than 4,000 feet above the level of the -sea; but by the terrible outburst which then took place the cone was -reduced in height by 400 feet, and a vast crater, which had a diameter -of nearly a mile, and a depth of nearly 1,000 feet (see fig. 13), was -formed at the top of the mountain. The enormous quantity of material -thus removed was either distributed over the flanks of the mountain, -or, when reduced to a finely comminuted condition, was carried by the -wind to the distance of many miles, darkening the air, and coating the -surface of the ground with a thick covering of dust. - -[Illustration: Fig. 13.--Crater of Vesuvius formed during the eruption -in 1822. (It was nearly 1 mile in diameter and 1,000 ft. deep.)] - -[Sidenote: EARLY HISTORY OF VESUVIUS.] - -The volcano of Vesuvius, although of somewhat insignificant dimensions -when compared with the grander volcanic mountains of the globe, -possesses great interest for the student of Vulcanology, inasmuch as -being situated in the midst of a thickly populated district and in -close proximity to the city of Naples, it has attracted much attention -during past times, and there is no other volcano concerning which we -have so complete a series of historical records. The present cone of -Vesuvius, which rises within the great encircling crater-ring of Somma, -has a height of about 1,000 feet. But there is undoubted evidence that -this cone, to the top of which a railway has recently been constructed -for the convenience of tourists, has been entirely built up during the -last 1,800 years, and, what is more, that during this period it has -been many times almost wholly destroyed and reconstructed. - -Nothing is more certain than the bet that the Vesuvius upon which the -ancient Romans and the Greek settlers of Southern Italy looked, was a -mountain differing entirely in its form and appearance from that with -which we are familiar. The Vesuvius known to the ancients was a great -truncated cone, having a diameter at its base of eight or nine miles, -and a height of about 4,000 feet. The summit of this mountain was -formed by a circular depressed plain, nearly three miles in diameter, -within which the gladiator Spartacus, with his followers, were -besieged by a Roman army. There is no evidence that at this time the -volcanic character of the mountain was generally recognised, and its -slopes are described by the ancient geographers as being clothed with -fertile fields and vineyards, while the hollow at the top was a waste -overgrown with wild vines. - -[Illustration: Fig. 14.--Crater of Vesuvius in 1756. (From a drawing -made on the spot)] - -But in the year 79 a terrible and unexpected eruption occurred, by -which a vast, crateral hollow was formed in the midst of Vesuvius, and -all the southern side of the great rim surrounding this crater was -broken down. Under the materials ejected during this eruption, the -cities of Pompeii, Herculaneum, and Stabiæ were overwhelmed and buried. - -Numerous descriptions and drawings enable us to understand how in the -midst of the vast crater formed in the year 79 the modern cone has -gradually been built up. Fresh eruptions are continually increasing the -bulk, or raising the height of the Vesuvian cone. - -The accompanying drawings made by Sir William Hamilton enable us to -understand the nature of the changes which have been continually -taking place at the summit of Vesuvius. The drawing fig. 14 shows the -appearance presented by the crater in the year 1756. - -[Illustration: Fig. 15.--The Summit of Vesuvius in 1767. (From an -original drawing.)] - -[Sidenote: VESUVIUS IN MODERN TIMES.] - -At this time we see that inside the crater a series of cones had been -built up one within the other from which lava issued, filling the -bottom of the crater and finding its way through a breach in its walls, -down the side of the cone. It is evident that the ejected materials -falling on the sides of the innermost cone would tend to enlarge the -latter till its sides became confluent with the cone surrounding it, -and if this action went on long enough, the crater would be entirely -filled up and a perfect cone with only a small aperture at the top -would be produced. But from time to time, grand and paroxysmal -outbursts have occurred at Vesuvius, which have truncated the cone, -and sometimes formed great, cup-shaped cavities, reaching almost to its -base, like that shown in fig. 13. - -In 1767 the crater of Vesuvius, as shown in fig. 15, contained a single -small cone in a state of constant spasmodic outburst, like that of -Stromboli. - -[Illustration: Fig. 16.--Summit of Vesuvius in 1848.] - -In 1843, we find that the crater of Vesuvius contained three such small -cones arranged in a line along its bottom as depicted in fig. 16. - -These drawings of the summit of Vesuvius give a fair notion of the -changes which have been continually going on there during the whole of -the historical period. Ever and anon a grand outburst, like that of -1822, has produced a vast and deep crater such as is represented in -fig. 13, and then a long continuance of quiet and regular ejections -has built up within the crater small cones like those shown in figs. -14, 15 and 16, till at last the great crater has been completely filled -up, and the cone reconstructed. - -[Illustration: Fig. 17.--Outlines of Vesuvius, showing its Form at -different periods of its history.] - -[Sidenote: CHANGES IN OUTLINE OF VESUVIUS.] - -In the series of outlines in fig. 17, we have endeavoured to illustrate -the succession of changes which has taken place in Vesuvius during -historical times. In the year 79 one side of the crater-wall of the -vast mountain-mass was blown away. Subsequent ejections built up the -present cone of Vesuvius within the great encircling crater-wall of -Somma, and the form of this cone and the crater at its summit have -been undergoing continual changes during the successive eruptions of -eighteen centuries. - -What its _future_ history may be we can only conjecture from analogy. -It may be that a long continuance of eruptions of moderate energy may -gradually raise the central cone till its sides are confluent with -those of the original mountain; or it may be that some violent paroxysm -will entirely destroy the modern cone, reducing the mountain to the -condition in which it was after the great outburst of 79. On the other -hand, if the volcanic forces under Vesuvius are gradually becoming -extinct (but of this we have certainly no evidence at present), the -mountain may gradually sink into a state of quiescence, retaining its -existing form. - -The series of changes in the shape of Vesuvius, which are proved by -documentary evidence to have been going on during the last 2,000 years, -probably find their parallel in all active volcanoes. In all of these, -as we shall hereafter show, the activity of the vents undergoes great -vicissitudes. Periods of continuous moderate activity alternate with -short and violent paroxysmal outbursts and intervals of complete rest, -which may in some cases last for hundreds or even thousands of years. -During the periods of continuous moderate activity, the crater of the -volcano is slowly filled up by the growth of smaller cones within it; -and the height of the mountain is raised. By the terrible paroxysmal -outbursts the mountain is often completely gutted and its summit blown -away; but the materials thus removed from the top and centre of the -mass are for the most part spread over its aides, so that its bulk -and the area of its base are thereby increased. During the intervals -of rest, the sides of the mountain which are so largely composed of -loose and pulverulent materials are washed downwards by rains and -driven about by winds. Thus all volcanoes in a state of activity are -continually growing in size every ejection, except in the case of those -where the materials are in the finest state of subdivision, adding to -their bulk; the area of their bases being increased during paroxysmal -outbursts, and their height during long-continued moderate eruptions. - -[Sidenote: DEVIATIONS FROM CONICAL FORM.] - -We have pointed out that the conical form of volcanic mountains is -due to the slipping of the falling materials over one another till -they attain the angle at which they can rest. There are, however, some -deviations from this regular conical form of volcanoes which it may be -well to refer to. - -The quantity of rain which falls during volcanic eruptions is often -enormous, owing to the condensation of the great volumes of steam -emitted from the vent. Consequently the falling lapilli and dust often -descend upon the mountain, not in a dry state but in the condition -of a muddy paste. Many volcanic mountains have evidently been built -up by the flow of successive masses of such muddy paste over their -surfaces. Some volcanic materials when mixed with water have the -property of rapidly 'setting' like concrete. The ancient Romans and -modern Italians, well acquainted with this property of certain kinds of -volcanic dust and lapilli, have in all ages employed this 'puzzolana,' -as it is called, as mortar for building. The volcanic muds have often -set in their natural positions, so as to form a rock, which, though -light and porous, is of tolerably firm consistency. To this kind of -rock, of which Naples and many other cities are built, the name of -'tuff' or 'tufa' is applied. A similar material is known in Northern -Germany as 'trass.' - -The cause of the 'setting' of puzzolana and tufa is that rain-water -containing a small proportion of carbonic acid acts on the lime in the -volcanic fragments, and these become cemented together by the carbonate -of lime and the free silica, which are thus produced in the mass. - -When a strong wind is blowing during a volcanic outburst, the materials -may be driven to one side of the vent, and accumulate there more -rapidly than on the other. Thus lop-sided cones are formed, such -as may frequently be observed in some volcanic districts. In areas -where constant currents of air, like the trade-winds, prevail, all -the scoria-cones of the district may thus be found to be unequally -developed on opposite sides, being lowest on those from which the -prevalent winds blow, and highest on the sides towards which these -winds blow. - -[Sidenote: ANGLE OF SLOPE IN VOLCANIC CONES.] - -The examination of any careful drawing, or better still of the -photograph, of a volcanic cone, will prove that the profile of such -cones is not formed by straight lines, but by curves often of a -delicate and beautiful character. The delineations of the sacred -volcano of Fusiyama, which are so constantly found in the productions -of Japanese artists, must have familiarised everyone with the elegant -curved lines exhibited by the profiles of volcanoes. The upper slope -of the mountain is comparatively steep, often exhibiting angles of -30° to 35°, but this steepness of slope gradually diminishes, till it -eventually merges in the surrounding plains. The cause of this elegant -form assumed by most volcanic mountains is probably two-fold. In the -first place we have to remember that the materials falling upon the -flanks of the mountain differ in size and shape, and some will rest -on a steeper slope than others. Thus, while some of the materials -remain on the upper part of the mountains, others are rolling outwards -and downwards. Hence we find that those cones which are composed of -uniform materials have straight sides. But in some cases, we shall -see hereafter, there has certainly been a central subsidence of the -mountain mass, and it is this subsidence which has probably given rise -to the curvature of its flanks. - -We have hitherto considered only the methods by which the froth or -foam, which accumulates on the surface of fluid lava, is dispersed. -But in many cases not only is this scum of the lava ejected from the -volcanic vent by the escaping steam, but the fluid lava itself is -extruded forcibly, and often in enormous quantities. - -The lava in a volcanic vent is always in a highly heated, usually -incandescent, condition. Seen by night, its freshly exposed surface -is glowing red, sometimes apparently white-hot. But by exposure to -the atmosphere the surface is rapidly chilled, appearing dull red by -night, and black by day. Many persons are surprised to find that a -flowing stream of lava presents the appearance of a great mass of rough -cinders, rolling along with a rattling sound, owing to the striking of -the clinker-like fragments against each other. When viewed by night, -the gleaming, red light between these rough, cindery masses betrays the -presence of incandescent materials below the chilled surface of the -lava-stream. - -No fact in connection with lavas is more striking than the varying -degrees of liquidity presented by them in different cases. While some -lava-streams seem to resemble rivers, the material flowing rapidly -along, filling every channel in its course, and deluging the whole -country around, others would be more fitly compared to glaciers, -creeping along at so slow a rate that the fact of their movement can -only be demonstrated by the most careful observation. Even when falling -over a precipice such lavas, owing to their imperfect liquidity, form -heavy, pendent masses like a 'guttering' candle, as is shown by fig. -18, which is taken from a drawing kindly furnished to me by Capt. S. P. -Oliver, R.A. The causes of these differences in the rate of motion of -lava-streams we must proceed to consider. - -[Illustration: Fig. 18.--Cascade of Lava tumbling over a cliff in the -Island of Bourbon.] - -[Sidenote: TEMPERATURE OF LAVA-STREAMS.] - -There can be no doubt that the temperature of lavas varies greatly in -different cases. This is shown by the fact that while some lavas are in -a state of complete fusion, similar to that of the slags of furnaces, -and like the latter, such lavas on cooling form a glassy mass, others -consist of a liquid magma in which a larger or smaller number of -crystals are found floating. In these latter cases the temperature of -the magma must be below the fusing-point of the minerals which exist -in a crystalline condition in its midst. It has indeed been suggested -that the whole of the crystals in lavas are formed during the cooling -down of a completely fused mass; but no one can imagine that the -enclosed crystals of quartz, felspar, leucite, olivine, &c., have -been so formed, such crystals being sometimes more than an inch in -diameter. The microscopic examination of lavas usually enables us to -discriminate between those complete crystals which have been formed at -great depths and carried up to the surface, and the minute crystalline -particles and microliths which have been developed in the glassy mass -during cooling. Crystals of the former class, indeed, exhibit abundant -evidence, in their liquid cavities and other peculiarities, that they -have not been formed by simple cooling from a state of fusion, but -under the combined action of heat, the presence of water and various -gases, and intense pressure. - -As we have already seen, the different lavas vary greatly in their -degrees of fusibility. The basic lavas, containing a low percentage -of silica, are much more fusible than the acid lavas, which contain -a high percentage of silica. When the basic lavas are reduced to a -complete state of fusion their liquidity is sometimes very perfect, as -is the case at Kilauea in Hawaii, where the lava is thrown up into jets -and fountains, falling in minute drops, and being drawn out into fine -glassy threads. On the other hand, the less fusible acid lavas appear -to be usually only reduced to the viscous or pasty condition, which -artificial glasses assume long before their complete fusion. Of this -fact I have found many proofs in the Lipari Islands, where such glassy, -acid lavas abound. In fig. 6 (page 43) a lava-stream is represented on -the side of the cone of Vulcano. - -[Sidenote: IMPERFECTLY FLUID LAVAS.] - -This lava is an obsidian--that is to say, it is of the add type and -completely glassy--but its liquidity must have been very imperfect, -seeing that the stream has come to a standstill before reaching the -bottom of a steep slope of about 35°. In fig. 19 there is given a side -view of the same stream of obsidian, from which it will be seen that -it has flowed slowly down a steep slope and heaped itself up at the -bottom, as its fluidity was not complete enough to enable it to move on -a slighter incline. An examination of the interior of such imperfectly -fluid lavas affords fresh proofs of the slow and tortuous movements -of the mass. Everywhere we find that the bands of crystallites and -sphærulites are, by the movement of the mass, folded and crumpled and -puckered in the most remarkable manner, as is illustrated in figs. 20 -and 21. Similar appearances occur again and again among the vitreous -and semi-vitreous acid lavas of Hungary. - -[Illustration: Fig. 19.--Lava-stream (obsidian) in the Island of -Vulcano showing the imperfect liquidity of the mass.] - -[Illustration: Fig. 20.--Interior of a Rhyolitic Lava-stream in the -Island of Lipari, showing broad sigmoidal folds produced by the slow -movements of the mass.] - -[Illustration: Fig. 21.--Interior of a Rhyolitic Lava-stream in the -Island of Lipari, showing the complicated crumplings and puckerings -produced by the slow movements of the mass.] - -[Sidenote: RATE OF MOVEMENT OF VESUVIAN LAVAS.] - -But, although the temperature of lava-streams and the fusibility of -their materials may in some cases account for their condition of -either perfect liquidity or viscidity, it is clear that in other -instances there must be some other cause for this difference. Thus -it has been found that at Vesuvius the lavas erupted in modern times -have all a striking similarity to one another in chemical composition, -in the minerals which they contain, and in their structure. They are -all basic lavas, which when examined by the microscope are seen to -consist of a more or less glassy magma, in the midst of which numerous -crystals of augite, leucite, olivine, magnetite, and other minerals -are scattered. Yet nothing can be more strikingly different than the -behaviour of the lavas poured out from Vesuvius at various periods. In -some cases the lava appears to be in such a perfectly liquid condition -that, issuing from the crater, it has been described as rushing down -the slope of the cone like a stream of water, and such exceedingly -liquid lavas have in some cases flowed to the distance of several -miles from the base of the mountain in a very short time. But other -Vesuvian lavas have been in such a viscid condition that their rate -of movement has been so extremely slow as to be almost imperceptible. -Such lava-streams have continued in movement during many years, but the -progress has been so slow (often only a few inches in a day) that it -could only be proved by means of careful measurements. - -If we examine some of these Vesuvian lavas which have exhibited such -striking differences in their rate of flow, we shall find that they -present equally marked differences in the character of their surfaces. -The lava-current of 1858 was a remarkable example of a slow-flowing -stream, and its surface, as will be seen in fig. 22, which is taken -from a photograph, has a very marked and peculiar character. A -tenacious crust seems to have formed on the surface, and by the further -motion of the mass this crust or scum has been wrinkled and folded -in a very remarkable manner. Sometimes this folded and twisted crust -presents a striking resemblance to coils of rope. Precisely similar -appearances may be observed on the surface of many artificial slags -when they flow from furnaces, and are seen to be due to the same -cause, namely, the wrinkling up of the chilled surface-crust by the -movement of the liquid mass below. Lavas which present this appearance -are frequently called 'ropy lavas'; an admirable example of them is -afforded in the lava-cascade of the Island of Bourbon represented in -fig. 18 (page 93). - -But lavas in which the rate of flow has been very rapid, exhibit -quite a different kind of surface to that of the ropy lavas. The -Vesuvian lava-stream of 1872 was remarkable for the rapidity of its -flow, and its surface presents a remarkable contrast to that of the -slow-moving lava of 1858. The surface of the lava-current of 1872 is -covered with rough cindery masses, often of enormous dimensions, and -it is exceedingly difficult to traverse it, as the ragged projecting -fragments tear the boots and lacerate the skin. The appearance -presented by this lava-stream is illustrated by fig. 23, which is also -taken from a photograph. - -[Illustration: Fig. 22.--Vesuvian Lava-stream of 1858, exhibiting the -peculiar 'Ropy' Surfaces of Slowly Moving Currents. - -(_From a Photograph._)] - -[Illustration: Fig. 23.--Vesuvian Lava-stream of 1872, exhibiting the -Rough Cindery Surfaces characteristic of Rapidly Flowing Currents. - -(_From a Photograph._)] - -[Sidenote: VESUVIAN LAVA-STREAM OF 1872.] - -Now it is found that those lava-streams which move slowly and present -ropy surfaces give off but little steam during their flow, while those -lava-streams which flow more rapidly and present a rough and cindery -appearance give off vast quantities of steam. The extraordinary amount -of vapour given off from the lava-streams which flowed from Vesuvius -in 1872 is illustrated in the photograph copied in fig. 5 (facing page -24), in which the three lava-currents are each seen to be surmounted -by enormous vapour-clouds rising to the height of several thousands -of feet above them, and mingling with the column that issued from -the central vent. By the escape of this enormous quantity of steam -the surface of the lava was thrown into rugged cindery projections, -and in some places little cones were formed upon it, which threw out -small scoriæ and dust. The quantity of vapour was, in fact, so great, -that little parasitical volcanoes were formed on the surface of the -lava-stream. Some of these miniature volcanoes were of such small -dimensions that they were carried away on boards to be employed as -illustrations in the lecture-rooms of the University of Naples. - -The arrangement of the materials forced out from fissures on the -surfaces of lava-streams by the disengaged vapours and gases depends -on the degree of fluidity of the lava, and the force of the escaping -steam-jets. In very viscous lavas the materials may issue quietly, -forming great concentric masses like coils of rope; such were described -by Mr. Heaphy as occurring in New Zealand (see fig. 24). - -[Illustration: Fig. 24.--Concentric Folds on mass of cooled Lava.] - -In other cases the lava, if somewhat more liquid, may in issuing -quietly without great outbursts of steam, accumulate in great -bottle-shaped masses, which have been compared to 'petrified -fountains.' Cases of this kind have been described by Professor Dana as -occurring on the slopes of Hawaii (see fig. 25). - -[Illustration: Fig. 25.--Mass of cooled Lava formed over a spiracle on -the slopes of Hawaii.] - -[Sidenote: MINIATURE CONES ON LAVA-STREAMS.] - -When the steam escapes with explosive violence from a spiracle -('bocca') on the surface of a lava-stream, minute cinder cones, -like those described as being formed in 1872, are the result. Fig. -26 represents a group of miniature cones thrown up on the Vesuvian -lava-stream of 1855: it is taken from a drawing by Schmidt. - -[Illustration: Fig. 26.--Group of small Cones thrown up on the Vesuvian -Lava-current of 1856.] - -Some of these appear like burst blisters or bubbles, while others -are built up of scoriaceous masses which have been ejected from the -aperture and have become united while in a semi-fluid condition. Other -examples of these spiracles or bocche on the surfaces of lava-currents -may be seen in the figs. 22 and 23, which are copied from photographs. - -The facts we have described all point to the conclusion that the -presence of large quantities of water imprisoned in a mass of lava -contributes greatly to its mobility. And this conclusion is supported -by so many other considerations that it is now very generally accepted -by geologists. The condition of this imprisoned water in lavas is one -which demands further investigation at the hands of physicists. It has -been suggested, with some show of reason, that the water may exist -in the midst of the red-hot lava as minute particles in the curious -'spheroidal condition' of Boutigny, and that these flash into steam as -the lava flows along. - -Lava, when extruded from a volcanic crater in a more or less completely -fluid state, flows down the side of the cone, and then finds its way -along any channel or valley that may lie in its course, obeying in its -movements all the laws of fluid bodies. The lava-currents thus formed -are sometimes of enormous dimensions, and may flood the whole country -for many miles around the vent. - -Lava-streams have been described, which have flowed for a distance of -from fifty to a hundred miles from their source, and which have had -a breadth varying from ten to twenty miles. Some lava-streams have a -thickness of 500 feet, or even more. These measures will give some -idea of the enormous quantities of material brought from the earth's -interior by volcanic action and distributed over its surface. The mass -of lava which flowed out during an eruption off Reykjanes in Iceland, -in the year 1783, has been calculated to be equal in bulk to Mont Blanc. - -There are many parts of the earth's surface, such as the Western Isles -of Scotland and the North-east of Ireland, the Deccan of India, and -large tracts in the Rocky Mountains, where successive lava-sheets have -been piled upon one another to the height of several thousands of feet, -and cover areas of many hundreds or even thousands of square miles. - -[Sidenote: FEATURES OF LAVA-STREAMS.] - -The more fusible basic lavas are as a general rule more liquid in -character than any others, and it is these very liquid lavas that are -usually found forming plateaux built up of successive lava-streams. The -less liquid lavas, like those of Hungary and Bohemia, are not usually -found flowing to such distances from the vent, but form dome-shaped -mountain-masses. - -Lava-streams usually exhibit in their upper and under surfaces a -scoriaceous texture due to the escape of steam from the upper surface, -portions of the cindery masses so formed falling off from the end of -the stream, and being rolled over by the stream so as to form its -base. The thickness of this scoriaceous upper and lower part of a -lava-stream varies according to the quantity of steam imprisoned in -it; but all thick lava-streams have a compact central portion which -is composed of hard, solid rock. Very good examples of the internal -structure of lava-streams may sometimes be examined in the sea-cliffs -of volcanic islands. In fig. 27 we have given a copy of a drawing made -while sailing round the shores of Vulcano. The scoriaceous portions of -lava-streams are sometimes employed, as at Volvic in the Auvergne, as -a building material, or as at Neidermendig in the Eifel and in Hungary -for mill-stones; the compact portions are employed for building and -paving, and for road metal. The rock of some of the modern lava-streams -of Vesuvius is largely quarried for paving the streets of Naples. - -This solid portion of the lava-streams in slowly cooling down from its -highly-heated condition undergoes contraction, and in consequence is -rent asunder by a number of cracks. Sometimes these cracks assume a -wonderfully regular arrangement, and the rock may be broken up into -very symmetrical masses. - -[Illustration: Fig. 27.--Natural section of a Lava-stream in the Island -of Vulcano, showing the compact central portion and the scoriaceous -upper and under surfaces.] - -[Sidenote: COLUMNAR STRUCTURE OF LAVAS.] - -If we imagine a great sheet of heated material, like a lava-stream, -slowly cooling down, it is evident that the contraction which must -take place in it will tend to produce fissures breaking up the mass -into prisms. A little consideration will convince us what the form of -these prisms must be. There are only three regular figures into which -a surface can be divided, namely, equilateral triangles, squares, and -regular hexagons; the first being produced by the intersection of -sets of six lines radiating at angles of 60° from certain centres; -the second by the intersection of sets of _four_ lines radiating from -centres at angles of 90°; and the third from sets of _three_ lines -radiating from centres at an angle of 120°. It is evident that a less -amount of contractile force will be required to produce the sets of -_three_ cracks rather than those of four or six cracks; or, in other -words, the contractile force in a mass will be competent to produce the -cracks which give rise to hexagons rather than those which form squares -or triangles. This is no doubt the reason why the prisms formed by the -cooling of lava, as well as those produced during the drying of starch -or clay, are hexagonal in form. - -The hexagonal prisms or columns formed by contraction during the -consolidation of lavas vary greatly in size, according to the rate of -cooling, the nature of the materials, and the conditions affecting the -mass. Sometimes such columns may be found having a diameter of eight -or ten feet and a length of five hundred feet, as in the Shiant Isles -lying to the north of the Island of Skye; in other cases, as in certain -volcanic glasses, minute columns, an inch or two in length and scarcely -thicker than a needle, are formed; and examples of almost every -intermediate grade between these two extremes may sometimes be found. -The largest columns are those which are formed in very slowly cooling -masses. - -The columnar structure is exhibited by all kinds of lava, and indeed -in other rock-masses which have been heated by contact with igneous -masses and gradually cooled. The rocks which display the structure in -greatest perfection, however, are the basalts. - -Mr. Scrope first called attention to the fact that the upper and lower -portions of lava-streams sometimes cool in very different ways, and -hence produce columns of dissimilar character. The lower portion of the -mass parts with its heat very slowly, by conduction to the underlying -rocks, while the upper portions radiate heat more irregularly into -the surrounding atmosphere. Hence we often find the lower portions of -thick lava-streams to be formed of stout, vertical columns of great -regularity; while the upper part is made up of smaller and less regular -columns, as shown in fig. 28. - -[Illustration: Fig. 28.--Section of a Lava-stream exposed on the side -of the river Ardèche, in the south-west of France.] - -The remarkable grotto known as Fingal's Cave in the Island of Staffa -has been formed in the midst of a lava-stream such as we have been -describing; the thick vertical columns, which rise from beneath the -level of the sea, are divided by joints and have been broken away by -the action of the sea; in this way a great cavern has been produced, -the sides of which are formed by vertical columns, while the roof is -made up of smaller and interlacing ones. The whole structure bears some -resemblance to a Gothic cathedral; the sea finding access to its floor -of broken columns, and permitting the entrance of a boat during fine -weather. Similar, though perhaps less striking, structures are found in -many other parts of the globe wherever basaltic and other lava-streams -exhibit the remarkable columnar structure as the result of their slow -cooling. Portions of basaltic columns are often employed for posts -by the road-sides, as in Central Germany and Bohemia, or for paving -stones, as in Pompeii and at the Monte Albano near Rome. - -[Illustration: Fig. 29.--Portion of a Basaltic Column from the Giant's -Causeway, exhibiting both the ball-and-socket and the tenon-and-mortise -structures.] - -[Sidenote: OTHER JOINT-STRUCTURES IN LAVAS.] - -Occasionally basaltic lava-streams exhibit other curious structures in -addition to the columnar. Thus some basaltic columns are found divided -into regular joints by equidistant, curved surfaces, the joints thus -fitting into one another by a kind of ball-and-socket arrangement. -Sometimes we find processes projecting from the angles of the curved -joint-surfaces, which cause the blocks to fit together as with a tenon -and mortise. This kind of structure is admirably displayed at the -Giant's Causeway, Co. Antrim, in the North of Ireland. A portion of a -basaltic column from this locality is represented in fig. 29. - -[Illustration: Fig. 30.--Vein of green Pitchstone, at Chiaja di Luna -in the Island of Ponza, breaking up into regular columns, and into -spherical masses with a concentric series of joints.] - -While the ordinary columnar structures are very common in basalts, -the ball-and-socket and tenon-and-mortise structures are exceedingly -rare. The question of the mode of origin of these remarkable structures -has given rise to much discussion, and the opinions of geologists and -physicists are by no means unanimous upon the subject. - -Sometimes we find masses of lava traversed by curved joints, and -occasionally we find curious combinations of curved and plane joints, -giving rise to appearances scarcely less remarkable than those -presented by the columns of the Giant's Causeway. Some of the more -striking examples of this kind have been described and explained by -Professor Bonney. - -[Illustration: Fig. 31.--Illustration of the 'Perlitic structure' in -glassy Rocks. - - a. Perlltic structure, as seen in a lava from Hungary. - b. The same structure, artificially produced in Canada Balsam - during cooling. -] - -[Sidenote: PERLITIC-STRUCTURE IN LAVAS.] - -In the Ponza Islands there occurs a remarkable example of a columnar -pitchstone, which is also traversed by a member of curved concentric -joints, causing the rock to break up into pieces like the coats of an -onion. This remarkable rock-mass is represented in fig. 30. - -A very similar structure is often seen in certain glassy lavas, when -they are examined in thin sections under the microscope. Such glassy -lavas exhibit the peculiar lustre of mother-of-pearly doubtless in -consequence of the interference of light along the cracks. Lavas -exhibiting this character are known to geologists as 'perlites.' The -perlitic structure has been produced artificially by Mr. Grenville Cole -in Canada Balsam, and by MM. Fonqué and Michel Lévy, in chemically -deposited silica. See fig. 31. - -A thick lava-stream must take an enormous period to cool down--probably -many hundreds or even thousands of years. It is possible to walk over -lava-streams in which at a few inches below the surface the rock is -still red-hot, so that a piece of stick is lighted if thrust into a -crack. Lava is a very bad conductor of heat, and loose scoriæ and -dust are still worse conductors. During the eruption of Vesuvius in -1872, masses of snow which were covered with a thick layer of scoriæ, -and afterwards by a stream of lava, were found three years afterwards -consolidated into ice, but not melted. The city of Catania is -constantly supplied with ice from masses of snow which have been buried -under the ejections of Etna. - -During the cooling down of lavas, the escape of steam and various -gases gives rise to the deposition of many beautiful crystalline -substances in the cavities and on the surfaces of the lava. Deposits -of sulphur, specular-iron, tridymite, and many other substances are -often thus produced, and the colour and appearance of the rock-masses -are sometimes completely disguised by these surface incrustations, or -by the decomposition of the materials of the lava by the action of the -add gases, and vapours upon it. - -[Sidenote: SINKING OF SURFACES OF LAVA-STREAMS.] - -Very frequently the surface of a lava-stream becomes solid, while the -deeper portions retain their fluid condition; under such circumstances -the central portions may flow away, leaving a great hollow chamber or -cavern. In consequence of this action, we not unfrequently find the -upper surface of a lava-current exhibiting a depression, due to the -falling in of the solidified upper portions when the liquid lava has -flowed away and left it unsupported, as in fig. 32. - -[Illustration: Fig. 32.--Transverse section of a Lava stream. (The -dotted line indicates the original surface.)] - - - - -CHAPTER V - -THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS. - - -Near the high-road which passes between the towns of Eger and -Franzenbad in Bohemia, there rises a small hill known as the Kammerbühl -(see fig. 33), which has attracted to itself an amount of interest -and attention quite out of proportion to its magnitude or importance. -During the latter part of the last century and the earlier years of -the present one, the fiercest controversies were waged between the -partisans of rival schools of cosmogony over this insignificant hill; -some maintaining that it originated in the combustion of a bed of coal, -others that its materials were entirely formed by some kind of 'aqueous -precipitation,' and others again that the hill was the relic of a small -volcanic cone. - -Among those who took a very active part in this controversy was -the poet Goethe, who stoutly maintained the volcanic origin of the -Kammerbühl, styling it 'a pocket edition of a volcano.' To Goethe -belongs the merit of having suggested a Very simple method by which the -controversies concerning this hill might be set at rest: he proposed -that a series of excavations should be undertaken around the hill, and -a tunnel driven right under its centre. - -[Illustration: Fig. 33.--The Kammerbühl of Kammerberg, Bohemia. - -(As seen from the south-west)] - -[Sidenote: THE KAMMERBÜHL.] - -The poet's friend, Count Caspar von Sternberg, determined to put this -project into execution. This series of excavations, which was completed -in 1837, has for ever set at rest all doubts as to the volcanic origin -of the Kammerbühl. A plug of basalt was found filling the centre of -the mass, and connected with a small lava-stream flowing down the side -of the hill; while the bulk of the hill was shown to be composed of -volcanic scoriæ and lapilli. The section fig. 34 will illustrate the -structure of the hill as revealed by these interesting excavations. - -[Illustration: Fig. 34.--Section of the Kammerbühl, in Bohemia. - -_a a._ Metamorphic rocks. _b._ Basaltic scoriæ. _c._ Solid plug -of basalt rising through the centre of the volcanic pile, _d d._ -Lava-stream composed of the same rock. _e e._ Alluvial matter -surrounding the old volcano. - -(The dotted lines indicate the probable former outline of the volcano.)] - -[Sidenote: VOLCANOES DISSECTED BY DENUDATION.] - -It can of course very seldom happen that actual mining operations, like -those undertaken in the case of the Kammerbühl, will be resorted to -in order to determine the structure of volcanic mountains. Geologists -have usually to avail themselves of less direct, but by no means less -certain, methods than that of making artificial excavations in order -to investigate the earth's crust. Fortunately it happens that what we -cannot accomplish ourselves, nature does for us. The action which we -call 'denudation' serves as a scalpel to dissect volcanic mountains -for us, and to expose their inner recesses to our view. Many portions -of the earth's surface are complete museums crowded with volcanic -'subjects,' exhibiting every stage of the process of dissection. In -some, rains and winds have stripped off the loose covering of cinders -and dust, and exposed the harder and more solid parts--the skeleton of -the mountain. In others, the work of destruction has proceeded still -further, and slowly wearing rivers or the waves of the sea may have cut -perfect, vertical sections of the mountain-mass. Sometimes the removal -of the materials of the volcanic mountain has gone on to such an extent -that its base and ground-plan are fully exposed. It only requires the -necessary skill in piecing together our observations on these dissected -volcanoes, in order to arrive at just views concerning the 'comparative -anatomy' of volcanoes. As the knowledge of the structure of animals -remained in the most rudimentary condition until the practice of -dissection was commenced, so our knowledge of volcanoes was likewise -exceedingly imperfect till geologists availed themselves of the -opportunities afforded to them of studying naturally dissected volcanic -mountains. - -In some cases we may find that the sea has encroached on the base -of a volcanic hill, till one half of it has been washed away, and -the structure of the mass to its very centre is exposed to our view. -Thus in fig. 6 (page 43), it will be seen that there lies in front of -Vulcano a peninsula called Vulcanello, consisting of three volcanic -cones, united at their base, with the lava-streams which have flowed -from them. One half of the cone on the left-hand side of the picture -has been completely washed away by the sea, and a perfect section -of the internal structure of the cone is exposed. The appearances -presented in this section are shown in the sketch, fig. 35. Some -portions of the face of this section are concealed by the heaps of -fragments which have fallen from it, but enough is visible to convince -us that three kinds of structures go to make up the cone. In the first -place, we have the loose scoriæ and lapilli, which in falling through -the air have arranged themselves in tolerably regular layers upon the -sides of the cone. - -[Illustration: Fig. 35.--Natural section or a Volcanic Cone in the -Island of Vulcano. - -_a._ Crater. _b b._ Lava-streams. _c._ Dykes which have clearly formed -the ducts, through which the lava has risen to the crater. _d d._ -Stratified volcanic scoriæ. _e._ Talus of fallen materials.] - -In the second place, we have lava-streams which have been ejected from -the crater or from fissures on the flanks of the cone, and flowed down -its sides. And thirdly, we find masses of lava filling up cracks in -the cone; these latter are called 'dykes.' Of these three kinds of -structures most volcanic mountains are built up, but in different -cases the part played by these several elements may be very unequal. -Sometimes volcanoes consist entirely of fragmentary materials, at -others they are made up of lavas only, while in the majority of -cases they have been formed by alternations of fragmentary and fluid -ejections, the whole being bound together by dykes, which are masses of -lava injected into the cracks formed from time to time in the sides of -the growing cone. - -If we direct our attention in the first place to the fragmentary -ejections, we shall find that they affect a very marked and peculiar -arrangement, which is best exhibited in those volcanic cones composed -entirely of such materials. - -[Sidenote: INTERNAL STRUCTURE OF VOLCANIC CONES.] - -Everyone who examines volcanoes for the first time will probably be -struck by the regular stratification of materials of which they are -composed. Thus the tuffs covering the city of Pompeii are found to -consist of numerous thin layers of lapilli and volcanic dust, perfectly -distinct from one another, and assuming even the arrangement which we -usually regard as characteristic of materials that have been deposited -from a state of suspension in water. The fragmentary materials in -falling through the air are sorted, the finer particles being carried -farther from the vent than the larger and heavier ones. The force -of different volcanic outbursts also varies greatly, and sometimes -materials of different character are thrown out during successive -ejections. These facts will be illustrated by fig. 36, which is a -drawing of a section exposed in a quarry opened in the side of the -Kammerbühl. In this section we see that the falling scoriæ have been -arranged in rudely parallel beds, but the regular deposition of these -has been interrupted by the ejection of masses of burnt slate torn from -the side of the vent, probably during some more than usually violent -paroxysm of the volcano. In those volcanoes which are built up of tuffs -and materials which have fallen in the condition of a muddy paste, the -perfect stratification of the mass is often very striking indeed, and -large cones are found built up of thin uniformly-spread layers of more -or less finely-divided materials, disposed in parallel succession. -Such finely-stratified tuff-cones abound in the district of the Campi -Phlegræi. - -[Illustration: Fig. 36.--Section in the side of the Kammerbühl, Bohemia. - -_a a._ stratified basaltic scoriæ. _b b._ Bands made up of fragments -of burnt slate. _c._ Stratified basaltic scoriæ. _d d._ Pseudo-dykes -occupying lines of fault.] - -[Sidenote: ARRANGEMENT OF FRAGMENTAL MATERIALS.] - -If, in consequence of any subterranean movements, fissures are produced -in the sides of the cones formed of fragmentary materials, these often -become gradually filled with loose fragments from the sides of the -fissure, and in this manner 'pseudo-dykes' are formed. An example of -such pseudo-dykes is represented in fig. 36, where the beds composing -the volcanic cone of the Kammerbühl are seen to have been broken across -or faulted, and the fissures produced in the mass have been gradually -filled with loose fragments. - -It is not difficult to imitate, on a small scale, the conditions which -exist at those volcanic vents from which only fragmentary materials -are ejected. If we take a board having a hole in its centre, into -which a pipe is inserted conveying a strong air-blast, we shall, by -introducing some light material like bran or sawdust into this pipe -cause an ejection of fragments, which will, when the board is placed -horizontally, fall around the orifice of the pipe and accumulate there -in a conical heap (fig. 37). It will be found necessary, as was shown -by Mr. Woodward, who performed the experiment before the Physical -Society, to adopt some contrivance, such as a screw, for forcing the -material into the air-pipe. If we alternately introduce materials of -different colours, like mahogany- and deal-sawdust into the pipe, these -materials will be arranged in layers which can be easily recognised, -and the mode of accumulation of the mass will be evident. By means of -a sheet of tin or cardboard we may divide this miniature volcanic cone -vertically into two portions, and if we sweep one of these away the -internal structure of the other half will be clearly displayed before -our eyes. - -In this way we shall find that the conical heap of sawdust with the -hole in its centre has a very peculiar and definite arrangement of its -materials. It is made up of a number of layers each of which slopes in -opposite directions, towards the centre of ejection and away from that -centre. These layers are thickest along the line of the circle where -the change in slope takes place, and they thin away in the direction of -the two opposite slopes. - -[Illustration: Fig. 37.--Experimental illustration of the mode of -Formation of Volcanic Cones composed of fragmental materials.] - -[Sidenote: CAUSE OF THIS ARRANGEMENT.] - -The cause of this peculiar arrangement of the materials is evident. -The sawdust thrown up by the air blast descends in a shower and tends -to accumulate in a circular heap around the orifice, the area of this -circular heap being determined by the force of the blast. Within this -circular area, however, the quantity of falling fragments is not -everywhere the same; along a circle surrounding the vent at a certain -distance, the maximum number of falling fragments will be found to -descend, and here the thickest deposit will take place. As this goes -on, a circular ridge will be formed, with slopes towards and away -from the centre of injection. As the ridge increases in height, the -materials will tend to roll down either one slope or the other, and -gradually a structure of the form shown in the figure will be piled up. -The materials sliding down the outer slope will tend to increase the -area of the base of the cone, while those which find their way down the -inner slope will fall into the vent to be again ejected. - -[Illustration: Fig. 38.--Natural section of a Tuff-cone forming the -Cape of Misenum, and exhibiting the peculiar internal Arrangement -characteristic of volcanoes composed of fragmentary materials.] - -Volcanic cones composed of scoriæ, dust, &c. are found to have exactly -the same internal structure as is exhibited by the miniature cone of -sawdust. The more or less regular layers of which they are made up dip -in opposite directions, away from and towards the vent, and thin out -in the direction of their dip (see fig. 38). In small cones the crater -or central cavity is of considerable size in proportion to the whole -mass, but as the cone grows upwards and outwards, the dimensions of -the crater remain the same, while the area of the base and the height -of the cone are continually increasing. This is the normal structure -of volcanic cones formed of fragmentary materials, though, as we shall -hereafter show, many irregularities are often produced by local and -temporary causes. - -[Illustration: Fig. 39.--Section of a small Scoria-cone formed within -the crater of Vesuvius in the year 1885, illustrating the filling-up of -the central tent of the cone by subsequent ejections.] - -In some cases the central vent of a volcanic scoria-cone may be -filled up by subsequent ejections. A beautiful example of this kind -was observed by Abich, in the case of a small cone formed within the -crater of Vesuvius in 1835, and is represented in fig. 39. - -[Illustration: Fig. 40.--Volcanic Cones composed of Scoriæ, and -breached on one side by the outflow of lava-currents.] - -[Sidenote: BREACHED CONES.] - -Many cones formed in the first instance of scoriæ, tuff, and pumice may -give rise to streams of lava, before the vent which they surround sinks -into a state of quiescence. In these cases, the liquid lava in the vent -gives off such quantities of steam that masses of froth or scoriæ are -formed, which are ejected and accumulate around the orifice. When the -force of the explosive action is exhausted, the lava rises bodily in -the crater, which it more or less completely fills. But, eventually, -the weaker side of the crater-wall yields beneath the pressure of the -liquid mass, and this part of the crater and cone is swept away before -the advancing lava-stream. Examples of such 'breached cones' abound in -Auvergne and many other volcanic districts (see fig. 40). A beautiful -example of a cone formed of pumice, which has been breached by the -outflow of a lava-stream of obsidian, occurs in the Lipari Islands, at -the Rocche Rosse. It is this locality which supplies the whole world -with pumice (see fig. 41). - -[Illustration: Fig. 41.--Campo Bianco, in the Island of Lipari. A -Pumice-cone breached by the Outflow of an Obsidian Lava-current.] - -It is often surprising to find how volcanic cones composed of loose -materials, such as tuffs, scoriæ, or pumice, retain their distinctive -forms, and even the sharpness of their outlines, during enormous -periods of time. Thus, in the scoria-cones which abound in the -Auvergne, and were, in all probability, formed before the historical -period, the sharp edges of the craters appear to have suffered scarcely -any erosion, and the cones are as perfect in their outlines as though -formed but yesterday. It is probable that the facility with which these -cindery heaps are penetrated by the rain which falls upon them is the -cause why they are not more frequently washed away. - -[Illustration: Fig. 42.--Volcanic Cones in Auvergne which have suffered -to some extent from atmospheric denudation.] - -Sometimes, however, scoria-cones are found reduced by atmospheric -waste to mere heaps of cinders, in which the position of the crater is -indicated only by a slight depression, as in fig. 42. - -[Sidenote: CONES COMPOSED OF LAVA.] - -When but little explosive action takes place at the volcanic vent, and -only fluid lava is ejected, mountains are formed differing very greatly -in character from the cones composed of fragmentary materials. - -If the lavas be of very perfect liquidity, like those erupted in the -Sandwich Islands, they flow outwards around the vent to enormous -distances. By the accumulation of materials during successive -outbursts, a conical mass is built up which has but a slight elevation -in proportion to the area of its base. Thus in Hawaii we find great -volcanic cones, composed of very fluid lavas, which have a height of -nearly 14,000 feet with a diameter of base of seventy miles. In these -Hawaiian mountains the slope of the sides rarely exceeds 6° to 8°. - -But if, on the other hand, the lavas be of much more viscid -consistency, the character of the volcanic cones which are produced by -their extrusion will be very different. The outwelling material will -tend to accumulate and heap itself up around the vent. By successive -ejections the first-formed shell is forced upwards and outwards, and -a steep-sided protuberant mass is formed, exhibiting in its interior -a marked concentric arrangement. Dr. Ed. Reyer, of Grätz, has devised -a very ingenious method for reproducing on a miniature scale the -characteristic features of these eruptions of viscid lavas. He takes a -quantity of plaster of Paris reduced to a pasty consistence, which he -forces through a hole in a board. The plaster accumulates in a great -rounded boss about the orifice through which it has been forced. If the -plaster have some colouring matter introduced into it, the mass, on -being cut across, will exhibit in the disposition of its colour-bands -the kind of action which has gone on during its extrusion, fig. 43. - -[Illustration: Fig. 43.--Experimental illustration of the Mode of -Formation of volcanic cones composed of viscid lavas.] - -[Illustration: Fig. 44.--The Grand Puy of Sarcoui, composed of -trachyte, rising between two breached scoria-cones (Auvergne).] - -There are many volcanic cones which exhibit clear evidence of having -thus been formed by the extrusion of a viscid mass of lava through -a volcanic fissure. Among such we may mention the domitic Puys of -Auvergne, fig. 44, many andesitic volcanoes in Hungary, the phonolite -hills of Bohemia, and the so-called 'mamelons' of the Island of -Bourbon. See figs. 45 and 46. When the interior of these masses is -exposed by natural or artificial sections, they are all found to -exhibit the onion-like structure which occurs in the plaster models. - -[Sidenote: INTERNAL STRUCTURE OF LAVA-CONES.] - -[Illustration: Fig. 45.--Volcanic Cone (Mamelon) composed of very -viscid lava. (Island of Bourbon.)] - -[Illustration: Fig. 46.--Another Mamelon in the Island of Bourbon, with -a crater at its summit.] - -But while some volcanoes are composed entirely of the fragmentary -ejections and others are wholly formed by successive outflows of lava, -the majority of volcanoes, especially those of larger dimensions, are -built up of alternations of these different kinds of materials. - -[Illustration: Fig. 47.--Cliff-section in the Island of Madeira, -showing how a composite volcano is built up of lava-streams, beds of -scoriæ, and dykes.] - -[Sidenote: NATURAL SECTIONS OF CONES.] - -The structure of these composite cones may be understood by an -inspection of the accompanying fig. 47, which shows the appearances -presented in a cliff on the coast of the Island of Madeira. We see -that the mass is made up of numerous layers of volcanic scoriæ, -alternating with sheets of lava. The latter, which are represented -in transverse section in the drawing, are seen to thin out on either -side, and to vary greatly in breadth. Besides the alternating masses -of scoriæ and the lava-sheets, there are seen in the section, bands -of a bright-red colour, which are represented in the drawing by black -lines. These are layers of soil, or volcanic dust, which, by the -passage of a lava-stream over their surface, have been burnt so as -to acquire a brick-red colour. These bands of red material, to which -the name of 'laterite' has been frequently applied, very commonly -occur in sections of composite volcanic cones. Crossing the whole of -the horizontally-disposed masses in the section, we find a number of -'dykes,' which are evidently great cracks filled with lava from below. -Some of these run vertically through the cliffs, others obliquely. In -some cases the lava, rising to fill a dyke, has flowed as a lava-stream -at the surface. Last of all, we must call attention to the fact that -the section exhibits evidence of great movements having taken place -subsequently to the accumulation of the whole of the materials. A -great crack has been produced, on one side of which the whole mass has -subsided bodily, giving rise to the phenomenon which geologists call a -'fault.' - -In the section, fig. 27, p. 104, copied from a drawing of a sea-cliff -in the Island of Vulcano, a transverse section of a lava-stream is -represented on a somewhat larger scale. The upper and under surface of -the lava-stream is seen to have a scoriaceous structure, but the thick -central mass is compact, and divided by regular joint-planes. This -section also illustrates the fact that, before the lava-stream flowed -down the sides of the mountain, a valley had been cut by meteoric -agencies on the flanks of the volcano, the dykes which traverse the -lower beds of tuff being abruptly truncated. - -In mountain ravines, upon the slopes of ancient volcanoes, and in the -cliffs of volcanic islands, we are often able to study the way in which -these great mountain masses are built up of alternating lava-currents, -beds of volcanic agglomerate, scoriæ, tuff and dust, and intersecting -dykes. In fig. 48, the features above described are illustrated by a -section in the sides of the great volcano of Mont Dore. - -[Illustration: Fig. 48.--Section seen at the cascade. Bains du Mont -Dore.] - -[Illustration: Fig. 49.--Section in the Island of Ventotienne, showing -a great stream of andesitic lava overlying stratified tuffs.] - -[Sidenote: SECTIONS IN THE PONZA ISLANDS.] - -In figs. 49, 50, 51, and 52, we have given drawings of portions of the -sea-cliffs in several of the Ponza Islands, a small volcanic group off -the Italian coast. - -[Illustration: Fig. 50.--Cliff on the south side of the Island of San -Stephano. - -_a._ Trachyte lava-stream, with a scoriaceous upper surface overlaid by -stratified tuffs, _b_.] - -[Illustration: Fig. 51.--The headland of Monte della Guardia, in the -Island of Ponza. - -_a._ Columnar trachyte. _b._ Stratified tuffs. _c._ Pumiceous -agglomerates. _d._ Dyke of rhyolite.] - -[Illustration: Fig. 52.--Western side of the same headland, as seen -from the north side of Luna Bay. - -_a._ Trachyte lava. _b._ Stratified tuffs. _c._ Dykes of rhyolite, with -their edges passing into pitchstone. _d._ Pumiceous agglomerate.] - -[Illustration: Fig. 53.--Sea-cliff at Il Capo, the north-east point of -Salina showing stratified agglomerates traversed by numerous dykes, the -whole being unconformably overlaid by stratified aqueous deposits.] - -Fig, 53 represents a cliff-section in the island of Salina, one of the -Liparis, exhibiting evidence that a series of volcanic agglomerates -traversed by dykes of Andesite have been denuded and covered by a -recent stratified deposit. - -[Sidenote: PART PLAYED BY DYKES IN CONE-BUILDING.] - -In the formation of these great composite cones, a minor but by no -means insignificant part is played by the dykes, or lava-filled -fissures, which are seen traversing the mass in all directions. That -dyke-fissures often reach the surface of a volcanic cone, and that -the material which injects them then issues as a lava-stream, is -illustrated by fig. 54. The formation of these cracks in a volcanic -cone, and their injection by liquid lava, must of course distend -the mountainous mass and increase its volume. If we visit the great -crater-walls of Somma in Vesuvius, and of the Val del Bove in Etna, -we shall find that the dykes are so numerous that they make up a -considerable portion of the mass. When the loose scoriæ and tuffs are -removed by denudation, these hard dykes often stand up prominently like -great walls, as represented in fig. 55. Even in such cases as these, -however, it is doubtful whether the bulk of all the dykes put together -exceeds one-tenth of that of the lavas and fragmentary materials. - -[Illustration: Fig. 54.--Section observed in the Val del Bove, Etna, -showing a basaltic dyke, from the upper part of which a lava-current -has flowed.] - -[Illustration: Fig. 55.--Basaltic Dykes projecting from masses of -stratified scoriæ in the sides of the Val del Bove, Etna.] - -Hence we are led by an examination of the internal structure of -volcanic mountains to conclude that scoriæ- and tuff-cones, and cones -formed of very liquid lavas, increase by an _exogenous_ mode of growth, -all new materials being added to them from without; in the cones formed -of very viscid lavas, on the other hand, the growth is _endogenous_, -taking place by successive accretions within it. The composite cones -owe their origin to both the _exogenous_ and the _endogenous_ modes of -growth, but in a much greater degree to the former than the latter. The -layers of scoriæ, tuff, and dust, and the successive lava-streams are -added to the mass from without, and the lava forming the dykes from -within it. - -[Sidenote: THEORY OF ELEVATION CRATERS.] - -There are doubtless cases in which, when a tuff-cone is formed, a mass -of very viscid lavas is extruded into its interior, and the mass is -distended like a gigantic bubble. But inasmuch as the very viscid lavas -do not appear to give rise to scoriæ to anything like the same extent -as the more liquid kinds, such 'cupolas,' as they have been called -by some German geologists, are probably not very numerous, and may -be regarded as constituting the exception rather than the rule. The -idea which was formerly entertained by some geologists that all great -volcanic mountains were formed of masses originally deposited in a -horizontal position, and subsequently blown up into a conical form, has -been effectually disposed of by the observations of Lyell and Scrope. - -The condition of the great fluid masses which underlie volcanic vents -is another point on which much light has been thrown by the study -of naturally-dissected volcanoes. In some cases, as was shown by -Hochstetter during his admirable researches among the New Zealand -volcanoes, the rising lavas form a great chamber for themselves in the -midst of a volcanic cinder-cone, taking the place of loose materials -which are re-ejected from the vent, or have been re-fused and absorbed -into the mass of lava itself. From this central reservoir of lava, -eruptions are kept up for some time, but when the volcano sinks -into a state of quiescence the lava slowly consolidates. In such -slowly solidified masses of lava, very beautiful groups of radiating -columns are often exhibited Northern Germany abounds with examples -of such basaltic masses, which have once formed the centres of great -cinder-cones; but in consequence of the removal of the loose materials -and the surrounding strata by denudation, these central reservoirs of -the volcanoes have been left standing above the surface, and exhibit -the peculiar arrangements of the columns formed in them during the -process of cooling. - -[Sidenote: INTRUSIVE LAVA-SHEETS.] - -But in the majority of the more solidly-built composite volcanoes no -such liquid reservoir can be formed within the volcanic cone itself. -Under these circumstances, the lavas, especially those of more liquid -character, tend to force passages for themselves among the rocks -through which they are extruded. Wherever a weak point exists, there -such lavas will find their way, and as the planes of stratification -in sedimentary rocks constitute such weak places, we constantly find -sheets of lava thus inserted between beds of aqueous origin. The areas -over which these intrusive sheets of rock sometimes extend may be very -great, but the more fusible, basic lavas (basalt, &c.) usually form -much more widely-spreading sheets than the less fusible, acid lavas. -In some cases these great intrusive sheets are found extending to a -distance of twenty or thirty miles from the centre at which they were -ejected, and they often follow the bedding of the strata with which -they are intercalated in so regular a manner, that it is difficult for -an observer to believe at first sight that they can have been formed in -the way which we have described. A closer examination will generally -reveal the fact that while these intrusive lava-sheets retain their -parallelism with the strata among which they have been intruded, over -considerable areas, yet they sometimes break across, or send offshoots -into them, as shown in fig. 56. In all cases, too, the rocks lying -above and below such sheets will be found to be more or less baked and -altered, and this affords a very convincing evidence of the intrusion -of the igneous mass between the strata so altered. - -[Illustration: Fig. 56.--Sheets of Igneous Rock (Basalt) intruded -between beds of sandstone, clay, and limestone. (Island of Skye.)] - -That in the case of most great volcanic mountains, or systems of -mountains, vast reservoirs of liquid lava must exist in the earth's -crust far below the surface, there can be little room for doubt. -Whether such fluid masses are in direct or indirect communication with -a great central reservoir, even supposing such to exist, is a totally -different question. In many cases the outburst of volcanoes in more or -less close proximity has been observed to take place simultaneously, -while in others the commencement of the eruption of one volcano has -coincided with the lapse into quiescence of another in its vicinity. -On the other hand, the remarkable case of the volcanoes of Hawaii -seems to indicate that two vents in close proximity may be supplied -from perfectly distinct reservoirs of lava. The active craters of -Mauna Loa and Kilauea are situated at the heights of 14,000 and 4,000 -feet respectively above the sea level; yet the former is sometimes -in a state of violent activity, with which the latter shows no signs -of sympathy whatever. We shall, in a future chapter, adduce evidence -that the liquid lavas in underground reservoirs may undergo various -stages of change in the enormous periods of time during which habitual -volcanic vents are supplied from them. - -We have already shown that the character assumed by a mass of fused -material in cooling varies greatly according as the cooling takes place -rapidly at the surface or slowly under enormous pressure. In the former -case a glassy base is formed containing a greater or smaller number -of crystallites or embryo crystals, in the latter the whole rock is -converted into a mass of fully-developed crystals. - -[Sidenote: CONSOLIDATION OF LAVAS AT GREAT DEPTHS.] - -The lavas which are poured out at the surface consist, as we have -seen, of a glassy magma in which a greater or smaller number of -crystals are found which have been borne up from below. The great -dykes and intrusive sheets consist for the most part of a mass of -small or imperfectly developed crystals in which a number of large and -perfectly formed crystals are embedded. Such rocks are said to have a -'porphyritic' structure. The rocks formed by the consolidation of the -liquid masses in the underground reservoirs are found to be perfectly -crystallised, the crystals impressing one another on every side and -making up the whole mass to the exclusion of any paste or magma between -them. The crystals in those rocks which have consolidated at these -vast depths exhibit evidence, in their enclosed watery solutions and -liquefied carbonic acid, of the enormous pressures under which they -must have been consolidated. The lavas, the more or less porphyritic -rocks of the dykes and sheets, and the perfectly crystalline (granitic) -rocks of the underground reservoirs pass into one another, however, by -the most insensible gradations. - -We sometimes find examples of volcanoes which, by the action of -denuding forces, have had their very foundations exposed to our view. -Such examples occur in the Western Isles of Scotland, in the Euganean -Hills near Padua in Northern Italy, and in many other parts of the -earth's surface. In these cases we are able to trace the ground-plan of -the volcanic pile, and to study the materials which have consolidated -deep beneath the surface in the very heart of the mountain. - -In studying these 'basal wrecks' of old volcanoes it is always -necessary to bear in mind that the appearance and general characters of -a volcanic rock may be completely disguised by chemical changes going -on within it. It is through want of attention to this fact that so many -mistakes were made by the Wernerian school of geologists who declared -that they could find no analogy between the basaltic rocks of the globe -and the products of active volcanoes, and were hence led to refer the -origin of the former to some kind of 'aqueous precipitation.' - -Many of the hard and crystalline marbles which are employed as -ornamental stones were originally loose masses of shells and corals, -as we easily perceive when we examine the polished faces. But these -incoherent heaps of organic _débris_ have been converted into a compact -and solid rock in consequence of the mass being penetrated by water -containing carbonate of lime in solution. Crystals of this substance -were deposited in every cavity and interstice of the mass, and thus -the accumulation of separate organisms was gradually transformed to a -material of great solidity and hardness. - -[Sidenote: FORMATION OF AMYGDALOIDS.] - -In precisely the same way loose heaps of scoriæ, lapilli, or pumice -may, by the passage through them of water containing various substances -in solution, have their vesicles filled with crystals, and thus be -converted into the hardest and most solid of rock-masses. Similarly -the scoriaceous portions of lava-streams have their vesicles filled -with crystalline substances deposited from a state of solution, and -are thus converted into a solid mass which may at first sight appear -to offer but little resemblance to the vesicular materials of recent -lava-streams. To these vesicular rocks which have their cavities filled -with crystalline substances geologists apply the name of amygdaloids -(_L. amygdalus_, an almond). The cavities in lava-rocks are usually -more or less elongated, owing to the movement of the mass while in -a still plastic state, and the crystalline materials filling these -cavities take the almond-like shape; hence the name. - -When the amygdaloids and altered fragmentary ejections of volcanoes -are studied microscopically, their true character is at once made -manifest. The exposure of faces of these altered volcanic rocks to the -weathering influences of the atmosphere, in many cases also causes -their true nature to be revealed, the crystalline materials filling -the interstices and vesicles of the mass are dissolved away by the -rain-water containing carbonic acid, and the rock regains its original -cavernous structure and appearance. But this repeated passage of water -through volcanic rock-masses may result in the removal of so large a -portion of their materials that the remainder crumbles down into the -condition of a clay or mud. - -In the basal wrecks of volcanoes, of which we have spoken, we usually -find only small and fragmentary remains of the great accumulations -of loose and scoriaceous materials which originally constituted the -bulk of the mountain mass. In the centre of the ground-plan of such -a denuded volcano we find great masses of highly crystalline or -granitic rock, which evidently occupy vast fissures broken through -the sedimentary or other rocks upon which the volcanic pile has been -reared. These highly crystalline rocks exhibit, as we have shown, -clear evidence of having been consolidated from a state of fusion -with extreme slowness and under enormous pressure, but their ultimate -chemical composition is identical with that of the lavas which have -been ejected from the volcano. - -When, as frequently happens, the volcano, after pouring out one kind -of lava for a certain period, has changed the nature of its ejections, -and given rise to materials of different composition, we find clear -evidence of the fact in studying the basal wreck or ground-plan of -the volcano. A great intrusive crystalline mass, of the same chemical -composition as the first-extruded lava, is found to be rent asunder and -penetrated by a similarly crystalline mass having the composition of -the lavas of the second period. Thus, in the volcanoes of the Western -Isles of Scotland, which are reduced by the action of denudation to -this condition of basal wrecks, we find that rhyolites, trachytes, and -andesites were ejected during the earlier periods of their history, and -basalts during the later periods. - -[Illustration: Fig. 57.--Plan of the Dissected Volcano of Mull, in the -Inner Hebrides.] - -[Illustration: - - _a_ Rocks on which the Volcano has been built up. - _b_ Great intrusive masses of acid and intermediate rocks. - _c_ Lara currents of basalt which have flowed from _d_. - _d_ Intrusive masses of gabbros & dolerite. - _e_ Lava currents which have flowed from _b_. - _f_ Volcanic tuffs and agglomerates. - -Fig. 58.--Section of the Volcano along the line _A B_.] - -[Sidenote: ANCIENT VOLCANO OF MULL.] - -We perceive on studying the ground-plan of these volcanoes that -great masses of granite, syenite, and diorite--the crystalline -representatives of the first-extruded lavas--are penetrated by -intrusions of gabbro--the granitic form of the later-ejected lavas. -These features are admirably illustrated by the ruined volcano now -constituting the Island of Mull, one of the Inner Hebrides, a plan -of which is given in fig. 57, and a section in fig. 58. This volcano -probably had a diameter at its base of nearly thirty miles, and a -height of from 10,000 to 12,000 feet, but is now reduced to a group of -hills few of which exceed 3,000 feet in height. - -From these great intrusive masses of highly crystalline rocks there -proceed in every direction great spurs or dykes, which are evidently -the radiating fissures formed during the outwelling of igneous -materials from below, injected by these fluid substances. The rock -forming these dykes is often less perfectly crystalline than that which -constitutes the centre of the mass, and we may indeed detect among the -materials of these dykes examples of every variety of structure, from -the perfectly crystalline granite to the more or less glassy substance -of lavas. Besides the vertical or oblique dykes we also find horizontal -sheets, which, passing from these central masses, have penetrated -between the surrounding strata, often, as we have seen, to enormous -distances. - -For the sake of simplicity, we have spoken of these ground-plans, or -basal wrecks of volcanoes, as constituting a flat plain; as a matter of -fact, however, the unequal hardness of the materials composing volcanic -mountains causes them to assume, under the influence of denuding -agencies, a very rugged and uneven surface. The hard crystalline -materials filling the central vent stand up as great mountain groups; -each large dyke, by the removal of the surrounding softer materials, is -left as a huge wall-like mass, while the remnants of lava-streams are -seen constituting a number of isolated plateaux. - -The great Island of Skye is the basal wreck of another volcano which -was also in eruption during Tertiary times; probably, many millions -of years ago. This immense volcano had originally a diameter at its -base of about thirty miles, and a height of 12,000 to 15,000 feet, -and must have been comparable to Etna or Teneriffe in its dimensions. -At the present time, there is nothing left of this vast pile but the -highly crystalline granites and gabbros filling up the great fissures -through which the eruption of igneous materials took place. These, worn -by denudation into rounded dome-like masses and wild rugged peaks, -constitute the Red Mountains and Coolin Hills of Skye, which rise to -the height of more than 3,000 feet above the sea-level. From these -great, central masses of crystalline rocks, innumerable radiating dykes -may be found rising through the surrounding rock-masses, with isolated -patches of the scoriæ and lapilli ejected from the volcano, which have -here and there escaped removal by denudation. Along what were the -outskirts of this great mountain-mass are found flat-topped hills, -built up of lava-streams, only small portions of which have escaped -removal by denudation. - -[Sidenote: RESERVOIRS BENEATH VOLCANOES.] - -But this wearing away of the structure of a volcanic cone by the -denuding forces may proceed even one stage farther, and we may then -have revealed for our inspection and study the mass of originally fluid -materials, from which one or more volcanoes have been fed, cooled and -consolidated in their original reservoir. There are many examples of -masses of granitic or highly crystalline rocks, having precisely the -same composition as the different varieties of lavas, which are found -lying in the midst of the sedimentary rocks, and sending off into -these rocks veins and dykes of the same composition with themselves. -No one who has carefully studied the appearances presented by volcanic -mountains in different stages of dissection, by the action of denuding -forces, can avoid recognising these great granitic masses as the cooled -reservoirs from which volcanoes have in all probability been supplied -during earlier periods of the earth's history. - -The eruption of these great masses of incandescent rock, impregnated -with water and acid gases, through strata of limestone, sandstone, -clay, coal, &c., may be expected to produce striking and wonderful -chemical changes in the latter. Nor are we disappointed in these -anticipations. Whenever we examine the sedimentary materials around -volcanic vents, we find that, in contact with the once-fused materials, -they everywhere exhibit remarkable evidences of the chemical action -to which they have been subjected. The limestones are converted into -statuary marble, the sandstones pass into quartzite, the days assume -the hardness and lustre of porcelain, while the coals have lost their -volatile ingredients and assumed a form like coke or graphite. And -these changes are found to extend in many cases to the distance of many -hundreds of yards from the planes of junction between the igneous and -the sedimentary materials. - -Among the most interesting effects resulting from the extrusion of -masses of incandescent rock, charged with water and various gases, -through beds of limestone, clay, sandstone, &c., we may mention the -production of those beautiful crystalline minerals which adorn our -museums and are so highly prized as gems. By far the larger part of -these beautiful minerals have been formed, directly or indirectly, by -volcanic agencies. - -These gems and beautiful minerals are, for the most part, substances -of every-day occurrence, which entirely owe their beauty to the -crystalline forms they have assumed. The diamond is crystallised -carbon, the ruby and sapphire are crystallised alumina, the amethyst -and a host of other gems are crystallised silica; and in almost all -cases the materials of gems are common and widely diffused, it is only -in their finely crystalline condition that they are rare and therefore -valuable. - -[Sidenote: FORMATION OF VOLCANIC MINERALS.] - -Crystals are formed during the slow deposition of a substance, either -by the evaporation of a liquid in which it is dissolved, by its -volatilisation, or its cooling from a state of fusion. In many cases it -can be shown that the formation of large and regular crystals is aided -if the work goes on with extreme slowness and under great pressure. -By sealing up various substances in tubes containing water which can -be kept at a high temperature, minute crystals of many well-known -minerals have been artificially formed by chemists. Part of the water -converted into steam has formed a powerful spring, which, reacting -upon the remainder of the liquid in the tube, has subjected it to -enormous pressure, and under these conditions of extreme pressure and -temperature, chemical actions take place of which we have no experience -under ordinary circumstances. The experiments of Mr. Hannay seem to -prove that when carbon is separated from certain organic substances -at a high temperature and under great pressure, it may crystallise in -the form of the diamond. And the recent discovery of diamonds in the -midst of materials filling old volcanic vents in South Africa seems -to show that this was in many cases the mode in which the gem was -originated. Even under the conditions which prevail at the earth's -surface, however, minute and unnoticed chemical actions taking place -during long periods of time, produce most remarkable results. This has -been well illustrated by M. Daubrée, who has shown that in the midst of -masses of concrete which the Romans built up around the hot springs of -Plombières and other localities, many crystalline minerals have been -formed, in the course of 2,000 years, by the action of the waters upon -the ingredients of the concrete. - -But most of the crystals of minerals which have been thus artificially -formed are of minute, indeed often of microscopic, dimensions. In the -underground reservoirs beneath volcanoes, however, we have all the -necessary conditions for the formation of crystals of minerals on a -far grander scale. High temperatures, pressures far greater than any -we can command at the earth's surface, the action of superheated steam -and many acid gases on the various constituents of both igneous and -sedimentary rocks, and, above all, time of almost unlimited duration; -these constitute such a set of conditions as may fairly be expected -to result in the formation of crystals, similar to those artificially -produced but of far greater size and beauty. - -If we visit those parts of the earth's surface where great masses of -fused volcanic rock have slowly cooled down in contact with sedimentary -materials, we shall not be disappointed in our expectations. Diamonds, -rubies, sapphires, emeralds, topazes, garnets, and a host of equally -beautiful, if less highly prized, crystalline substances, are found in -such situations, lying in the subterranean chemical laboratories in -which they have been formed, but now, by the action of denuding forces, -revealed to our view. - -In some cases it is not necessary to penetrate to these subterranean -laboratories in order to find these beautiful gems and other -crystallised minerals; for the steam jets which issue from volcanic -fissures carry up fragments of rock torn from the side of the vent, -and in the cavities and fissures of such ejected masses beautiful -crystallised products are often found. Such rock-fragments containing -minerals finely crystallised are found abundantly on the flanks of -Vesuvius and other active volcanoes, and among the materials of the -Laacher See and other extinct volcanoes. - -[Sidenote: FORMATION OF MINERAL-VEINS.] - -But it is not only the finely crystallised minerals and gems which -we owe to volcanic action. The various metallic minerals have nearly -all been brought from deep-seated portions of the earth's crust and -deposited upon the sides of rock-fissures by the agency of the same -volcanic forces. It is these forces which have, in the first instance, -opened the cracks through the solid rock masses; and, in the second -place, have brought the metallic sulphides, oxides, and salts--either -in fusion, in solution, or in a vaporised condition--from the -deep-seated masses within the earth, causing them to crystallise upon -the sides of the fissures, and thus form those metallic lodes and veins -which are within reach of our mining operations. - -There is still one other important class of minerals which owe the -existence, though indirectly, to volcanic agencies. The cavities of -igneous rocks, especially the vesicles formed by the escape of steam, -constitute, when filled with water, laboratories in which complicated -chemical reactions take place. The materials of the lava are gradually -dissolved and re-crystallised in new combinations. By this means the -most beautiful examples of such minerals as the agates, the onyxes, -the rock-crystals, the Iceland-spars, and the numerous beautiful -crystals classed together as 'Zeolites' have been formed. No one can -visit a large collection of crystalline minerals without being struck -with the large number of beautiful substances which have thus been -formed as secondary products from volcanic materials. - - - - -CHAPTER VI. - -THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS - - -From what has been said in the preceding chapters it will be seen that -while some of the materials ejected from volcanic vents are, by the -movements of the air and ocean, distributed over every part of the -face of the globe, another, and by far the larger, part of the matter -so ejected, accumulates in the immediate vicinity of the vent itself. -By this accumulation of erupted materials, various structures are -built up around the orifices from which the ejections take place, and -the size and character of these structures vary greatly in different -cases, according to the quantity and nature of the ejected materials, -and the intensity of the eruptive forces by which they were thrown from -the orifice. We shall proceed in the present chapter to notice the -chief varieties in the forms and characters of the heaps of materials -accumulated round volcanic vents. - -These heaps of materials vary in size from masses no bigger than a -mole-heap up to mountains like Etna, Teneriffe, and Chimborazo. The -size of volcanic mountains is principally determined by the conditions -of the eruptive action at the vent around which they are formed. If -this action exhausts itself in a single effort, very considerable -volcanic cones, like the Monte Nuovo with many similar hills in its -vicinity, and the Puys of Auvergne, may be formed; but if repeated -eruptions take place at longer or shorter intervals from the same vent, -there appears to be scarcely any limit to the size of the structures -which may, under such conditions, be formed. It is by this repeated -action from the same volcanic vent going on for thousands or even -millions of years, that the grandest volcanic mountains of the globe -have been built up. Such volcanoes have sometimes a diameter at their -base of from 30 to 100 miles, and an elevation of from 10,000 to 25,000 -feet. - -The _form_ of volcanic mountains is determined in part by the nature -of the materials ejected, and in part by the character of the eruptive -action. - -From what has been said in the preceding chapter, it will be gathered -that the volcanoes built up by ejections of fragmentary materials -differ in many striking particulars from those formed by the outwelling -of lavas from volcanic vents. In a less degree, the volcanoes composed -of the same kind of volcanic materials also vary among themselves. - -[Sidenote: CHARACTERS OF SCORIA-CONES.] - -When masses of scoriæ in a semi-fluid condition are thrown to only a -little distance above the volcanic vent, so that they have not time to -assume a perfectly solid condition before they fall round the vent, the -rugged masses of lava unite to form heaps of most irregular shape. In -such cases, the falling fragments being in a semi-plastic state, stick -to the masses below, and do not tend to roll down the sides of the -heap. Irregular heaps of such volcanic scoriæ abound on the surfaces -of lava-streams, being piled up around each 'bocca' or vent which the -steam-jets escaping from the lava-currents form at their surfaces. -Such irregular accumulations of scoriæ were observed on the lavas of -Vesuvius during the eruptions of 1822, 1855, and 1872, and have also -been described in the case of many other volcanoes. In fig. 26 (p. 101) -we have given representations of a group of such irregular scoria-cones -which was observed by Schmidt on the Vesuvian lava of 1855. It will -be seen from this drawing that there is scarcely any limit to the -steepness of the sides of such scoria-heaps, in which the materials are -in an imperfectly solidified condition when they reach the ground. - -But in the majority of cases, the scoriæ ejected from volcanic vents -are thrown to a great height, and are in a more or less perfectly -solidified condition when they fell to the ground again. In such cases -the fragments obey the ordinary mechanical laws of falling bodies, -rolling and sliding over one another, till they acquire a slope which -varies according to the size and form of the fragments. In this way -the great conical mounds are formed which are known as 'cinder-cones,' -or more properly as 'scoria-cones.' Scoria-cones usually vary in the -slope of their sides from 35° to 40° and may differ in size from mere -monticules to hills a thousand feet or more in height. Scoria-cones -of this character abound in many volcanic districts, as the Auvergne, -where they may be numbered by thousands. The materials forming such -scoria-cones vary in size from that of a nut to masses as large as a -man's head, and fragments of even larger dimensions are by no means -uncommon. - -When the lava in a volcanic vent is perfectly glassy, instead of being -partially crystalline in structure, we find not scoriæ but pumice -ejected. In such cases, as in the Lipari Islands for example, we see -cones entirely built up of pumice. Such pumice-cones resemble in -the angle of their slope (see fig. 41, facing p. 124), the ordinary -scoria-cones, but are of a brilliant white colour, appearing as if -covered with snow. - -[Sidenote: PRESERVATION OF SCORIA-CONES.] - -Ordinary scoriæ are usually of a black colour when first ejected, but -after a short time the black oxide of iron (magnetite) which they -contain, attracts the oxygen of the air and moisture, and assumes the -reddish-brown colour of iron-rust. Under such circumstances the heaps -of black material gradually acquire the red-brown colour which is -characteristic of so many of the scoria-cones around Etna, and in the -Auvergne and the Eifel. The moisture of the air, and the rain falling -upon these loose cindery heaps, cause them to decompose upon their -surfaces; the action is facilitated by the growth of the lower forms -of vegetation, such as mosses and lichens, and thus at last a soil is -produced on the surfaces of these conical piles of loose materials -which may support an abundant vegetation. Cinder- or scoria-cones are -not uncommonly found retaining in a most perfect manner their regular, -conical form, the lips of their craters being sharp and unbroken as if -the cone were formed but yesterday, while their slopes may nevertheless -be covered with a rich soil supporting abundant grass and forest-trees. -It may at first sight seem difficult to understand how a loose mass of -scoriæ could have so long withstood the action of the rain and floods, -retaining so perfectly its even slopes and sharp ridges. A little -consideration will, however, convince us that it is the very loose and -pervious nature of the materials of which scoria-cones are composed, -which tends to their perfect preservation. The rain at once sinks into -their mass, before it has time to form rivulets and streams which would -wear away their surfaces and destroy the regularity of their outlines. - -Scoria- and pumice-cones are frequently found to be acted upon by acid -vapours to such an extent that the whole of the materials is reduced -to a white pulverulent mass. In these cases the oxides of iron and the -alkalis have united with the sulphuric or hydrochloric or carbonic -acids, the compounds being carried away in solution by the rain-water -falling on the mass; the materials left are silica, the hydrated -silicate of alumina, and hydrated sulphate of lime (gypsum), all of -which are of a white colour. - -Cinder- or scoria-cones, and pumice-cones, are often found raised -by the action of winds to a greater elevation on one side than the -other, in the manner already described. One side of the cone is often -seen to be more or less completely swept away by an outwelling stream -of lava, and thus breached cones are formed (see fig. 40, p. 123). -Not unfrequently we find a number of cones which are united more or -less completely at their bases, as in Vulcanello (fig. 6, p. 43), the -several vents being so near together that their ejections have mingled -with one another. Cones composed entirely of fragmentary materials -often show an approach to the beautifully curved slopes which we have -described as being so characteristic of volcanoes, as may be seen in -fig. 41, facing p. 124. In the case of scoria- and pumice-cones this -curvature is probably due to the rolling downnwards and outwards of the -larger fragments. - -We have already pointed out that with the scoriæ there are often -ejected fragments torn from the sides of the volcanic vents. Sometimes -such fragments are so numerous as to make up a considerable portion of -the mass of the volcanic cones. Thus in the Eifel we find hills, of -by no means insignificant size, completely built up of small scoriæ -and broken fragments of slate torn from the rocks through which the -volcanic fissures have been opened. Occasionally we see that few or no -scoriæ have been ejected, and the volcanic vents are surrounded simply -by heaps of burnt slate. - -The smaller fragmentary materials ejected from volcanic vents--such -as lapilli and dust--rest in heaps, having a different angle of slope -from those formed by scoriæ. In many cases, as we have seen, such -finely-divided materials descend in the condition of mud, which flows -evenly over the surface of the growing cone and consolidates in beds of -very regularly stratified 'tufa' or 'tuff.' - -[Sidenote: CHARACTERS OF TUFF-CONES.] - -The 'tuff-cones' thus formed differ in many important respects from the -scoria-cones already described. The slope of their sides varies from -15° to 30°, and is almost always considerably less than in scoria- and -pumice-cones. The tuff-cones undergo much more rapid degradation from -rain and moisture than do the scoria-cones; for, though the materials -of the former 'set,' as we have seen, into a substance of considerable -hardness, yet this substance, being much less pervious to water than -the loose scoria heaps, permits of the formation of surface-streams -which furrow and wear away the sides of the cones. Sometimes the sides -of the crater are found to be almost wholly removed by atmospheric -denudation, and only a shallow depression is found occupying the -site of the crater; such a case is represented in fig. 59. We not -unfrequently find the whole slopes of such cones to be traversed by a -series of radiating grooves passing from the summit to the base of the -mountains, these channels being formed by water, which has collected -into streams, flowing down the slopes of the mountains. The volcanic -cone, under these circumstances, frequently presents the appearance of -a partially opened umbrella. Owing to the impervious character of the -materials composing tuff-cones, their craters are frequently found to -be occupied by lakes. - -[Illustration: Fig. 59.--Summit of the volcano of Monte Sant' Angelo in -Lipari exhibiting a crater with walls worn down by denudatioh.] - -Tufas have usually a white or yellowish-brown colour, and these are the -colours exhibited by the cones composed of this material before they -become covered by vegetation. Tufas scoriæ, and lavas usually crumble -down to form a very rich soil, and many of the choicest wines are -produced from grapes grown on the fertile slopes of volcanic mountains. -When, however, as not unfrequently happens, the materials are finely -divided and incoherent, they are so easily driven about by the winds -that cultivation of any kind is rendered almost impossible. In the -Islands of Stromboli and Vulcano the gardens have to be surrounded by -high fences to prevent them from being overwhelmed by the ever-shifting -masses of volcanic sand. - -[Sidenote: CHARACTERS OF LAVA-CONES.] - -There are some cones which are composed in part of scoriæ and in part -of tufa. Hence we are sometimes at a loss whether to group them with -the one class of cones or the other. But in the majority of cases, -scoria- and tuff-cones present the sufficiently well-marked and -distinctive characters which we have described. - -Lava-cones differ quite as greatly in their forms as do the cones -composed of fragmentary materials, the variations being principally -determined by the degree of liquidity of the lavas. - -We sometimes find that outwelling masses of lava, when issuing in small -quantities from a vent, accumulate in cauliflower-shaped masses, or -sometimes in the form of a column, or bottle. Professor J. D. Dana -describes many such fantastically-formed masses of lava as being found -in Hawaii, one of which is represented in fig. 25 (p. 100). - -When the lava issues from the vent in great quantities it tends to -flow on all sides of it, and to build up a great conical heap above -the orifice. If the lava be very liquid it flows to great distances, -resting at a very slight slope. Thus we find that the volcanoes of -Hawaii have been built up of successive ejections of very liquid lava, -which have formed cones having a slope of only 6° to 8°, but of such -enormous dimensions that the diameter of their bases is seventy miles -and their height 14,000 feet. - -[Illustration: Fig. 60.--Outlines of Lava-cones. - -1. Mauna Loa, in Hawaii. Composed of very fluid lava. 3. The -Schlossberg of Teplitz, Bohemia. Composed of very imperfectly fluid or -viscid lava.] - -If, on the other hand, the lava be viscid, or very imperfectly liquid -in character, it tends to accumulate immediately around the vent; fresh -ejections force the first extruded matter outwards, in the manner so -well illustrated by Dr. Reyer's experiments, and at last a more or less -steep-sided bulbous mass is formed over the vent. Such bulbous masses, -composed of imperfectly fluid lavas, occur in many volcanic districts, -and constitute hills of considerable size. From the tendency of matters -thus extruded to choke up the vents, however, these volcanoes composed -of viscid lavas cannot be expected to attain the vast dimensions -reached by some of those composed of very liquid lavas. The difference -in the forms of lava-cones composed of very fluid or of somewhat -viscid materials is illustrated in fig. 58. When the interior of such -steep-sided volcanic mountains composed of viscid materials is exposed -by the action of denuding forces, the peculiar internal structure we -have described is displayed by them. In the Chodi-Berg of Hungary, -a great bulbous mass of andesitic rock, this endogenous structure is -admirably displayed. It is also well seen in the excavation of the hill -of the Grand Sarcoui, a similar mass, composed of altered trachyte, -which has been erupted in the midst of a scoria-cone in the Auvergne. -See fig. 44 (p. 126). - -[Sidenote: CHARACTERS OF COMPOSITE CONES.] - -Most of the great volcanic mountains of the globe belong to the -class of 'composite cones,' and are built up by alternate ejections -of fluid lava and fragmentary materials. The slope of the sides in -such composite cones is subject to a wide range of variation, being -determined in part by the degree of liquidity of the lavas, in part -by the nature of the fragmentary materials ejected, and in part by -the proportions which the fragmentary and lava-ejections bear to one -another. - -But there is another set of causes which tends to modify the form -and character of these composite, volcanic cones. As we have already -pointed out, the sides of such cones are liable to be rent asunder from -time to time, and the fissures so produced are injected with masses of -liquid lava from below. These fissures, rent in the sides of volcanic -cones, often reach the surface and eruptive action takes place, giving -rise to the formation of a cone, or series of cones, upon the line -of the fissure (fig. 61). Such small cones thrown up on the flanks -of a great volcanic mountain are known as 'parasitic cones'; though -subordinate to the great mountain mass, they may be in themselves of -considerable dimensions. Among the hundreds of parasitic cones which -stud the flanks of Etna, there are some which are nearly 800 feet in -height. - -[Illustration: Fig. 61.--Diagram illustrating the formation of -Parasitic Cones along lines of fissure formed on the flanks of a great -volcanic mountain.] - -[Illustration: Fig. 62.--Outline of Etna, as seen from Catania.] - -[Sidenote: FORMATION OF PARASITIC CONES.] - -The building up of parasitic cones upon the flanks of a volcanic -mountain tends, of course, to destroy its regular conical form. This -may be well seen in Etna, which, by the accumulation of materials -upon its flanks, has become a remarkably 'round-shouldered' mountain. -(See figs. 62 and 63.) At the same time it must be remembered that -materials erupted from the central vent tend to fill up the hollows -between these parasitic cones, and thus to restore to the mountain its -regularly conical form. - -[Illustration: Fig. 63.--Outline of Etna, as seen from the Val del -Bronte.] - -[Illustration: Fig. 64.--Plan of the Volcano forming the Island of -Ischia. - - _a, a, a._ The semi-circular crater-ring of Epomeo. - _b, c, d._ Lava-currents which have flowed from the principal crater. - _e, f, g, h._ Plateaux formed by ancient lava-currents. - _k._ Montagnone. } - _l._ Monte Rotaro. } - _m._ Monte Tabor. } Parasitic cones and craters on the slopes - _n._ Castiglione. } of the mountain. - _o._ Lago di Bagno. } - _p._ The Cremate. } - _r._ Lava-stream of the Arso, which flowed from the Cremate in 1301. - _x, x, x._ Raised beaches on the shores of the island, showing that it - has recently undergone elevation. -] - -The Island of Ischia is a good example of a great volcanic cone the -flanks of which are covered with numerous small parasitic cones. While -the great central volcano has evidently been long extinct, and one -side of its crater-wall is completely broken down, some of the small -parasitic cones around its base have been formed within the historical -period--one of them as recently as the year 1302. Fig. 64 is a plan of -the Island of Ischia, showing the numerous parasitic cones scattered -over the slopes of the principal cone. - -[Illustration: Fig, 65.--A primary Parasitic Cone with a secondary one -at its base--Ischia. - -_a._ Monte Rotaro. _b._ Monte Tabor. _c._ Lava-stream flowing from the -latter.] - -In one case we find that a parasitic cone, the Monte Rotaro, has itself -a similar smaller cone, which is parasitic to it, at its foot; this -secondary parasitic cone gives off a small lava-stream of trachyte, -which has flowed down to the sea. (See fig. 65.) - -[Illustration: Fig. 66.--Scoria-cone near Auckland, New Zealand, with a -lava-current flowing from it. - -The strata beneath the volcanic cone are exposed in the sea-cliff, and -exhibit proofs of depression having taken place.] - -[Illustration: Fig. 67.--Section of rocks below the ancient triassic -volcano of Predazzo in the Tyrol. - -The position of the strata _a b c_, etc., indicates a central -subsidence.] - -[Sidenote: SUBSIDENCE BENEATH VOLCANIC VENTS.] - -Most great volcanic mountains exhibit a tendency towards a subsidence -of their central portions, which may take place either during or -subsequently to their period of activity. When we examine the strata -upon which a volcano has been built up, but which are now exposed to -our study by denuding forces, we usually find that they incline towards -the centre of the eruptive activity. (See figs. 66 and 67.) Two causes -may contribute to bring about this result. In the first instance, -we must remark that the piling up of materials around the volcanic -vent causes the subjacent strata to be subjected to a degree of -pressure far is excess of that which acts upon the surrounding rocks. -And secondly, it must be borne in mind that the continual removal -of material from below the mountain must tend to the production of -hollows, into which the overlying strata will sink. The effect of this -central subsidence is to give to the flanks of volcanic cones those -beautifully curved outlines which constitute so striking a feature in -Vesuvius (see fig. 17, p. 87), Fusiyama (see fig. 77, No. 1, facing p. -178), and many other volcanic mountains. - -There seems, at first sight, to be scarcely any limit to the dimensions -which these great composite volcanic cones may attain: the lateral -eruptions tending to enlarge the area of the base of the mountain, -and, by the injection of the fissures, to knit together and strengthen -its structure, while the central eruptions continually increase the -elevation of the mass. Great, however, as is the force which is -concerned in the production of our terrestrial volcanoes, it has its -limits; and, at last, the piling up of materials will have gone on -to such an extent, that the active forces beneath the volcano are no -longer competent either to raise materials to the elevated summit of -the mountain or to tear asunder its strengthened and fortified flanks. -Under these circumstances, the volcanic forces, if they have not -already exhausted themselves, will be compelled to find weak places in -the district surrounding the volcano, at which fissures may be produced -and the phenomena of eruption displayed. - -[Sidenote: SHIFTING OF VOLCANIC FOCI.] - -Some volcanic cones exhibit evidence that during the series of -eruptions by which they have gradually been built up, the centre of -volcanic action has shifted to another point within the mountain. Thus -Lyell has shown, in the case of Etna, that during the earlier periods -in the history of the mountain the piling up of materials went on -around a centre which is now situated at a distance of nearly four -miles from the present focus of eruptive activity. Some of our old -British volcanoes, of which the denuded wrecks exist in the Western -Isles of Scotland, show similar evidence of a shifting of the axis of -eruption. - -One of the most conspicuous features of a volcanic cone is the great -depression or crater found at its summit. In describing the internal -structure of volcanic cones, we have seen how these craters are -produced and acquire their inverted conical form, by the slipping and -rolling back of materials towards the centre of eruptive action. - -Almost all volcanic cones exhibit craters, but in those which are -formed entirely by the outwelling of viscid lavas the central -depression is often slight and inconspicuous, and occasionally -altogether wanting. It frequently happens, however, that eruptive -action has ceased at the centre of a volcano, and its summit-crater -may by denudation be entirely destroyed, while new and active craters -are formed upon its flanks. Stromboli furnishes us with an admirable -example of this kind (see fig. 1, facing p. 10). Other volcanoes may -exhibit several craters, one at the summit of the mountain and others -upon its flanks. Of this we find a good example in Vulcano (fig. 6, p. -43). - -[Illustration: Fig. 68.--Cotopaxi (19,600 feet), as seen from a -distance of ninety miles.] - -When a volcano has been built up by regular and continuous eruptions -from the same volcanic vent, the size of the crater remains the same, -while the volcano continually grows in height and in the diameter of -its base. The size of the crater will be determined by the eruptive -force at the volcanic centre, the size of the mountain by the duration -of the volcanic activity and the quantity of material ejected. In the -earliest stage of its history, such a volcano will resemble Monte -Nuovo, which has a crater reaching down almost to the base of the -mountain; in the later stages of its history, such a volcano will -resemble Cotopaxi (fig. 68) and Citlaltepetl (fig. 69), in which the -crater, though of far greater absolute dimensions than that of Monte -Nuovo, bears but a small proportion to the vast cone at the summit of -which it is situated. - -[Illustration: Fig. 69.--Citlaltepetl, or the Pic d'Orizaba, in Mexico -(17,370 feet), as seem from the forest of Xalapa.] - -[Sidenote: ORIGIN OF VOLCANIC CRATERS.] - -In the great majority of volcanoes, however, eruptive action does not -go on by any means regularly and continuously, but terrible paroxysmal -outbursts occur, which suddenly enlarge the dimensions of the crater to -an enormous extent. - -In the year 1772, there occurred a volcanic eruption in the Island -of Java, which is perhaps the most violent and terrible that has -happened within the historical period. A lofty volcanic cone, called -Papandayang, 9,000 feet high, burst into eruption, and, in a single -night, 30,000,000,000 cubic feet of materials were thrown into the -atmosphere, falling upon the country around the mountain where no less -than forty villages were buried. After the eruption, the volcano was -found to have been reduced in height from 9,000 to 5,000 feet, and -to present a vast crater in its midst, which had been formed by the -ejection of the enormous mass of materials. - -Many similar cases might be cited of the removal of a great part of a -mountain-mass by a sudden, paroxysmal outburst. In some cases, indeed, -the whole mass of a mountain has been blown away during a terrific -eruption, and the site of the mountain is now occupied by a lake. This -is said to have been the case with the Island of Timor, where an active -volcano, which was visible from a distance of 300 miles at sea, has -entirely disappeared. - -The removal of the central portion of great volcanic mountains by -explosive action, gives rise to the formation of those vast, circular, -crater-rings of which such remarkable examples occur in many volcanic -districts. These crater-rings present a wall with an outer slope -agreeing with that of the volcanic cone of which they originally formed -a part, but with steep inner cliffs, which exhibit good sections of the -beds of tuff, ash, and lava with the intersecting dykes of which the -original volcano was built up. Near Naples, one of these crater-rings, -with sloping outer sides and steep inner ones, is employed to form the -royal game-preserve of Astroni, the only entrance to the crater being -closed by gates. - -[Sidenote: FORMATION OF CRATER-LAKES.] - -As these crater-rings are usually composed of materials more or -less impervious to water, they often become the site of lakes. The -beautiful circular lake of Laach, in the Rhine Provinces, with the -numerous similar examples of Central Italy--Albano, Nemi, Bracciano, -and Bolsena--the lakes of the Campi Phlegræi (Agnano, Avernus, &c.), -and some similar lakes in the Auvergne, may be adduced as examples of -crater-rings which have become the site of lakes. - -[Illustration: Fig. 70.--Lac Paven, in the Auvergne. - -_a._ Scoriæ. _b._ Basalt.] - -One of the most beautiful of the crater-lakes in the Auvergne is -Lac Paven (fig. 70), which lies at the foot of a scoria-cone, Mont -Chalme, and is itself surrounded by masses of ejected materials. The -crater-lake of Bagno, in Ischia (fig. 71), has had a channel cut -between it and the sea, so that it serves as a natural harbour. The -lake of Gustavila, in Mexico (fig. 72), is an example of a crater-lake -on a much larger scale. - -In many of these crater-rings the diameter of the circular space -enclosed by them is often very great indeed as compared with the height -of the walls. - -[Illustration: Fig. 71.--The crater-lake called Lago del Bagno, in -Ischia, converted into a harbour.] - -[Illustration: Fig. 72.--Lake of Gustavila, in Mexico. - -(The terraces round the lake have been artificially formed.)] - -[Sidenote: DIMENSIONS OF CRATER-LAKES.] - -Two of the largest crater-rings in the world are found in Central -Italy, and are both occupied by lakes, the circular forms of which -must strike every observer. The Lago di Bracciano, which lies to -the north-west of Rome, is a circular lake six and a half miles in -diameter, surrounded by hills which at their highest point rise to the -height of 1,486 feet above the sea, while the surface of the waters -of the lake is 640 feet above the sea-level. The Lago di Bolsena is -somewhat less perfectly circular in outline than the Lago di Bracciano; -it has a length from north to south of ten-and-a-quarter miles and a -breadth from east to west of nine miles; the surface of the waters of -this lake is 962 feet above that of the waters of the Mediterranean. -The lake of Bolsena, like that of Bracciano, is surrounded by hills -composed of volcanic materials; the highest points of this ring of -hills rise to elevations of 684, 780, and 985 feet respectively above -the waters of the lake. - -In these great circular lakes of Bolsena and Bracciano, as well as in -the smaller ones of Albano, Nemi, and the lakes of Frascati in the same -district, the vast circular spaces enclosed by them, the gradual outer -slope of the ring, and the inner precipices which bound the lake, all -afford evidence of the explosive action to which they owe their origin. - -But while the vast crater-rings we have mentioned are frequently found -to be occupied by lakes, there are many other similar crater-rings -which remain dry, either from the materials of which they are composed -being of more pervious character, or from rivers having cut a channel -through the walls of the crater, in this way draining off its waters. - -Thus in the Campi Phlegræi, while we have the craters of Agnano and -Avernus forming complete circular lakes, Astroni has only a few -insignificant lakelets on its floor, and the Pianura, the Piano di -Quarto, which have each a diameter of three or four miles, with many -others, remain perfectly dry. In the vicinity of the great crater-lakes -of Central Italy we find the crater-ring of the Vallariccia, which has -evidently once been a lake but is now drained, its floor being covered -with villages and vineyards. - -[Sidenote: CRATER-RINGS SURROUNDING CONES.] - -A comparison of these vast crater-rings leads us to the conclusion that -in the majority of cases, if not in every instance, they are composed -almost entirely of volcanic tuff and dust. In the case of the more -solidly-built composite volcanic cones, the volcanic forces, as we have -seen, produce fissures in the mass, and along these fissures parasitic -cones are thrown up, the tension of the mass of imprisoned vapours -below the mountain being thus from time to time relieved. But in the -case of a volcanic cone composed of loose fragmentary materials, such -temporary relief is impossible. The cracks, as soon as they originate, -will be filled up and choked by the falling in of materials from above -and at their sides. In this way the eruptive action will be continually -repressed, till at last the imprisoned vapours acquire such a high -state of tension that the outburst, when it occurs, is of the most -terrible character, and the whole central mass of the volcano is blown -into the air. It may often seem surprising that the ejection of such -vast masses of material from the centre of a volcanic cone does not -effect more in the way of raising the height of the crater-walls. But -it must be remembered that, in the case of craters of such vast area, -the majority of the ejected materials must fall back again within -its circumference. By repeated ejections these materials will at last -be reduced to such an extreme state of comminution that they can be -borne away by the winds, and spread over the country to the distance of -hundreds or thousands of miles. After great volcanic outbursts enormous -areas are thus found covered with fine volcanic dust to the depth of -many inches or feet. - -[Illustration: Fig. 73.--Peak of Teneriffe in the Canary Islands -(12,182 ft.), surrounded by great crater-rings.] - -Sometimes, as in the case of the Lago di Bracciano, the eruptive forces -appear to have entirely exhausted themselves in the prodigious outburst -by which the great crater was produced. But in other cases, as in that -of the Lago di Bolsena, the eruptive action was resumed at a later -date, and small tuff-cones were thrown up upon the floor of the crater; -these now rise as islands above the surface of the lake. In other -cases, again, the eruptive action was resumed after the formation of -the great crater-ring, with such effect that bulky volcanic cones were -built up in the midst of the crater-ring which surrounds them like a -vast wall; examples of this are exhibited in the extinct volcanoes of -Rocca Monfina and Monte Albano. Some of the grandest volcanoes of the -globe, such as Teneriffe (fig. 73), the volcanoes of Mauritius and -Bourbon (figs. 74 and 75), and many others that might be cited, are -thus found to be surrounded by vast crater-rings. Vesuvius itself is -surrounded by the crater-ring of Somma (fig. 76). - -[Illustration: Fig. 74.--The volcano of Bourbon, rising in the midst of -a crater-ring four miles in diameter.] - -[Illustration: Fig. 75.--The volcano of Bourbon, as seen from another -point of view, with three concentric crater-rings encircling its base.] - -[Sidenote: BASALTIC CONES IN TRACHYTIC CRATER-RINGS.] - -This formation of cone within crater, often many times repeated, is -very characteristic of volcanoes. The craters mark sudden and violent -paroxysmal outbursts, the cones are the result of more moderate but -long-continued ejection. Sometimes, as at Vesuvius in 1767 (fig. 15, -p. 85), we find a nest of craters and cones which very strikingly -exemplifies this kind of action. - -[Illustration: Fig. 76.--Vesuvius, as seen from Sorrento, half -encircled by the crater-ring of Somma.] - -We shall point out, hereafter, that at most volcanic centres the -ejection of trachytic lavas precedes that of the basaltic lavas. Now it -is these trachytic lavas which principally give rise to the formation -of the light lapilli of which tuff-cones are formed. Hence it is that -we so frequently find, as in the case of Vesuvius, Rocca-Monfina, and -many other volcanoes, that a great crater-ring, largely composed of -tuffs, encloses a cone built up of more basic lavas. - -In fig. 77 we have shown by a series of outline sections the various -forms assumed by volcanoes in consequence of the different kinds of -eruptive action going on in them:-- - -1. Is an outline of Fusiyama, an almost perfect cone, with a small -crater at its summit. The sides of this volcano admirably illustrate -the beautiful double curves characteristic of volcanic cones. - -2. Hverfjall in Iceland, a volcanic cone with a large crater, reaching -almost to its base. - -3. The crater-lake of Bracciano, in which the area of the crater is out -of all proportion to the height of the crater-walls. - -4. Rocca-Monfina, in Southern Italy, a tuff-cone of large dimensions, -in the midst of which an andesitic lava-cone has been built up. - -5. Teneriffe, in the Canary Islands, in which a perfect volcanic cone -has been built up in the centre of an encircling crater-ring. - -6. Vulcano, in the Lipari Islands, in which, by the shifting of the -centre of volcanic activity along a line of fissure, a series of -overlapping volcanic cones has been produced. - -[Illustration: Fig. 77.--Outlines of various Volcanoes, illustrating -the different relations of the craters to cones.] - -[Sidenote: SUBMARINE VOLCANOES.] - -While speaking of the varieties of form assumed by volcanic cones and -craters, we must not forget to notice the effects which are produced -by denuding forces upon them. In the case of submarine volcanoes, like -the celebrated island called by the English Graham Isle, by the French -Isle Julie, and by the Germans the Insel Ferdinandez (fig. 78), which -was thrown up off the coast of Sicily in 1831, it was evident that -volcanic outbursts taking place at some depth below the level of the -sea gradually piled up a cone of scoriæ with a crater in its midst. -By constant accessions to its mass, this scoria-cone was eventually -raised above the sea-level, but the action of the waves upon the loose -materials soon destroyed the crater-walls and eventually reduced the -island to a shoal. It is evident that in all cases in which eruptions -take place beneath the sea-level, and the loose materials are exposed -during their accumulation to the beating of the sea-waves, the form -of the volcanic cone so produced will be greatly modified by the -interaction of the two sets of opposed causes, the eruptive forces from -below and the distributive action of the sea-waves. - -[Illustration: Fig. 78.--Island thrown up in the Mediterranean Sea in -July and August 1881. - -(The view was taken in the month of September, after the sides of the -crater had been washed away by the waves.)] - -Craters when once formed are often rent across, along the line of the -fissure above which they are thrown up. Thus the crater of Vesuvius was -in 1872 rent completely asunder on one side, so that it was possible -to climb through the fissure thus produced and reach the bottom of the -crater. Streams flowing down the sides of the crater, and escaping -through such a rent, may in the end greatly modify the form and -disguise the characters of a volcanic crater. Of this kind of action we -have a striking example in the Val del Bove of Etna. - -Volcanoes, as we shall point out in the sequel, are after their -extinction frequently submerged beneath the waters of the ocean. The -sea entering the craters, eats back their cliff-like sides and enlarges -their areas. Such denuded waters are called 'calderas,' the channels -into them 'barrancos.' - -Sometimes the action of the waves upon a partially submerged volcano -has led to the cutting back of its slopes into steep cliffs, at the -same time that the crater-ring is enlarged. In such cases we have left -a more or less complete rocky ring, composed of alternating lavas and -fragmentary materials. Of such a ruined crater-ring, the Island of St. -Paul in the South Atlantic affords an admirable example. - -When the action of denudation has gone still further, all the lavas and -tuffs composing the cone may be completely removed and nothing left but -masses of the hard and highly-crystalline rocks which have cooled down -slowly in the heart of the volcano. An example of this kind is afforded -to us by St. Kilda, the remotest member of the British Archipelago. - -But although the majority of volcanic craters are clearly formed by -explosive action, there are some craters, like those of Kilauea in -Hawaii, which probably owe their origin to quite a different set of -causes. In this case the explosive action at the vent is but slight, -and the crater, which is of very irregular form, appears to have -originated in a fissure, which has been slowly enlarged by the liquid -lavas encroaching upon and eating away its sides. Such craters as -these, however, appear to be comparatively rare. - -Besides the great volcanic mountains composed of lava, scoriæ, tuff and -ash, there are other structures which are formed around volcanic vents -even when these do not eject molten rock-masses. The water which issues -in these cases either as steam or in a more or less highly heated -condition frequently carries materials in suspension or solution, and -these sometimes accumulate in considerable quantities around the vent. - -[Sidenote: FORMATION OF MUD-VOLCANOES.] - -When fissures are formed in the midst of loose argillaceous materials, -such as are frequently produced by the decomposition of volcanic rocks, -the waters which issue through them are sometimes so charged with muddy -matter that this accumulates to form cones having all the general -characters of volcanic mountains, and which occasionally rise to the -height of 250 feet. The gases and vapours which issue from these -'mud-volcanoes' are those which are known to be emitted from volcanic -vents at which the action going on is not very intense. Daubeny and -others have suggested that these mud-volcanoes may be the result of -actions which have little or no analogy with those which take place -at ordinary volcanic vents, and that the combustion of subterranean -beds of sulphur and similar causes would be quite competent to their -production. But inasmuch as these mud-volcanoes are almost always -situated in regions in which the more powerful volcanic action has -only recently died away, and the gases and vapours emitted by them are -very similar in character to those which issue from volcanoes, there -does not appear to be any good reason for doubting that they should be -classed as truly volcanic phenomena. - -Mud-volcanoes are found in Northern Italy near Modena, in Sicily near -Girgenti, on the shores of the Sea of Azof and the Caspian, in Central -America, and in other parts of the globe. The gas frequently escapes -from them with such violence that mud is thrown into the air to the -height of several hundreds of feet. Sometimes this gas is inflammable, -consisting of sulphuretted hydrogen, hydrogen, or some hydrocarbons, -and these gases occasionally take fire, so that true flames issue from -these mud-volcanoes. In other cases the mud-volcanoes appear to be -formed by either hot or cold springs containing large quantities of -suspended materials, and the liquid mud issues from the vent without -any violent eruptive action. - -[Illustration: Fig. 79.--Sinter-cones surrounding the orifices of -Geysers. - -1. Basin of the Great Geyser, Iceland. 2. Hot spring cone. 3. Old -Faithful. 4. The Great Geyser. 5. Liberty Cap. (2, 3, 4 and 5 are in -the Yellowstone Park district of the Rocky Mountains.)] - -[Sidenote: FORMATION OF SINTER-CONES.] - -The soluble materials which waters issuing from volcanic vents deposit -on their sides are chiefly silica and carbonate of lime. - -Hot springs, whether intermittent or constant, often contain large -quantities of silica in solution. The solution of this silica is -effected, at the moment of its separation from combination with the -alkali or alkaline earths, during the decomposition of volcanic rocks, -and is favoured by the presence of alkaline carbonates in the water, -and the high temperature and the pressure under which it exists in the -subterranean regions. When the water reaches the surface and, being -relieved from pressure, begins to cool down the silica is deposited. -By this deposited silica the basins around the geysers of Iceland are -formed. Sometimes conical structures are built up around the vents of -hot springs by the deposition of silica from their waters. Examples -of this kind abound in the National Park of Colorado, where they have -received fanciful names, such as the Beehive, Liberty Gap, &c. This -deposited silica is known to geologists as sinter. The forms of some of -the structures which surround the orifices of geysers is shown in fig. -79. The 'Liberty Cap' is an extinct geyser-cone fifty feet high and -twenty feet in diameter. - -Hot and cold springs rising in volcanic regions are often highly -charged with carbonic acid, and in passing through calcareous rocks -dissolve large quantities of carbonate of lime. Upon exposure to the -atmosphere, the free carbonic acid escapes and the carbonate of lime -is deposited in the form known as 'travertine.' Such springs occur -in great numbers in many volcanic regions. In the Auvergne great -rock-masses occur formed of carbonate of lime deposited from a state -of solution and taking the form of natural aqueducts and bridges. In -Carlsbad the numerous hot springs have deposited masses of pisolitic -rock (Strudelstein) which have filled up the whole bottom of the -valley, and upon these deposits the town itself is mainly built. -In Central Italy the deposits of travertine formed by calcareous -springs are of enormous extent and thickness: St. Peter's and all the -principal buildings of Rome being constructed of this travertine or -'Tibur-stone.' - -[Sidenote: FORMATION OF SINTER-TERRACES.] - -When springs charged with silica or carbonate of lime rise upon the -slope of a hill composed of loose volcanic materials, they give rise -to the remarkable structures known as sinter- and travertine-terraces -(see fig. 80). The water flowing downwards from the vent forms a -hard deposit upon the lower slope of the hill, while the continual -deposition of solid materials within the vent tends to choke it up. -As a new vent cannot be forced by the waters through the hard rock -formed below, it is originated a little higher up. Thus the site of the -spring is gradually shifted farther and farther back into the hill. As -deposition takes place along the surfaces over which this water flows, -terraces are built up enclosing basins. Of structures of this kind we -have remarkable examples in the sinter-terraces of Rotomahana in New -Zealand and the travertine-terraces of the Gardiner's River in the -Yellowstone Park district of the Rocky Mountains. - -[Illustration: Fig. 80.--Diagram illustrating the mode of formation of -Travertine and Sinter Terraces on the sides of a hill of tuff.] - - - - -CHAPTER VII. - -THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOLCANIC CENTRES. - - -That a volcanic vent, when once established, may display intense -activity during enormous periods of time, there cannot be the smallest -reason for doubting; for the accumulation of materials around some -existing volcanic centres must certainly have been going on during -many thousands, perhaps millions, of years. To us, whose periods of -observation are so circumscribed, it may therefore at first sight -appear a hopeless task to trace the 'life-history of a volcano,' to -discover the stages of its development, and to indicate the various -episodes which have occurred during the long periods it has been in -existence. But when it is remembered that we have the opportunity of -studying and comparing hundreds of such volcanoes, exhibiting every -varying phase of their development, we shall see that such an attempt -is by no means so unpromising as it at first sight appears to be. In -the present chapter, we shall give an account of the results which have -already been obtained by inquiries directed to this object. - -[Sidenote: CYCLES OF VOLCANIC PHENOMENA.] - -There is not the smallest room for doubt that during the past history -of our globe, exhibitions of subterranean energy have occurred at many -different parts of its surface. There is further evidence that at the -several sites where these displays of the volcanic forces have taken -place, the succession of the outbursts has run through a regular cycle, -gradually increasing in intensity to a maximum, and then as gradually -dying away. - -A little consideration will show that the first portion of this cycle -of events is the one which it is most difficult to examine and study. -The products of the earlier and feeble displays of volcanic activity, -at any particular centre, are liable to be destroyed, or masked, during -the ejection of overwhelming masses of materials in the later stages -of its more matured energy. That the feeble displays of volcanic -force now exhibited in some localities will gradually increase in -intensity in the future, and eventually reach the grandest stage of -development, there can be no reason for doubting. But, unfortunately, -we are quite unable to discriminate these feeble manifestations, which -are the embryonic stages in the development of grand exhibitions of -the volcanic forces, from slight outbursts which die away and make no -farther sign. - -From what has been proved concerning the true nature of volcanic -action, however, it is certain that the first step towards the -exhibition of such action, at any particular locality, must be the -production of an aperture in the earth's crust. Only by means of -such an aperture can the vapours, gases, and rocky materials reach -the surface, and give rise to the phenomena there displayed. There is -reason to believe that all such apertures are really of the nature of -fissures, or cracks, which have been opened through the superjacent -strata by the efforts of the repressed subterranean forces. - -Some recent writers have, it is true, endeavoured to draw a distinction -between what they call 'fissure-eruptions,' and eruptions taking -place from volcanic cones. But all volcanic outbursts are truly -'fissure-eruptions'--the subterranean materials finding their way to -the surface through great cracks, which, in a more or less vertical -position, traverse the overlying rock-masses. It is true that in many -cases portions of these cracks soon get choked up, while other portions -become widened, and the volcanic energy is concentrated at such spots. -Thus the materials ejected from these fissures are usually emitted in -greatest quantities at one or more points along the fissure, and a -single great volcanic vent, or a row of smaller vents, is established -upon the line at which the fissure reaches the surface. - -We have seen that the amount of explosive action taking place at -different volcanic vents varies according to the proportion of -imprisoned water contained in the lava. In the cases where there is -much explosive action, vast accumulations of scoriæ, lapilli, and -dust take place, and cones of great size are built up; but in those -cases where the explosive action is small the lavas flow quietly from -the vent, and only small scoriæ-cones are thrown up, these being -probably soon swept away by the lava-currents themselves or by denuding -agencies. But both kinds of eruption have equal claims to be called -'fissure-eruptions.' - -[Sidenote: FORMATION OF VOLCANIC FISSURES.] - -In the expansive force of great masses of imprisoned vapour, we have -a competent cause for the production of the fissures through which -volcanic outbursts take place. Such fissures are found traversing the -rocks lying above volcanic foci, and often extending to distances of -many miles, or even hundreds of miles, from the centres of activity. -Some of these cracks are found to be injected with fused materials from -below, others have been more or less completely filled with various -minerals that have been volatilized, or carried by superheated waters -from the deeper regions of the earth's crust. That many of the cracks -thus produced in the superjacent rocks, by the heaving forces of -imprisoned vapour seeking to escape, never reached the surface, we have -sufficient proof in many mining regions. - -If we now transfer our attention from the deeper portions of the -earth's crust to the surface, we can well understand how the attempts -of the imprisoned vapours to force a passage for themselves through the -solid rock-masses would lead to shocks and jars among the latter. Each -of these shocks or jars would give rise, in the surrounding portions -of the earth's crust, to those vibrations which we know as earthquakes. -The close connection between most earthquakes and volcanic phenomena is -a fact that does not admit of the smallest doubt; and though it would -be rash to define all earthquakes as 'uncompleted efforts to establish -a volcano,' yet, in the efforts of the repressed subterranean forces to -find a vent by the production of fissures in the overlying rock-masses, -we have a cause competent to the production of those shocks which are -transmitted to such enormous distances as waves of elastic compression. - -We have seen that the production of the fissure upon which the small -volcano of Monte Nuovo was thrown up was preceded by a succession -of earthquakes, which for a period of over two years terrified the -inhabitants of the district, and might have warned them of the -coming event. In the same manner, doubtless, the period before the -appearance of volcanic phenomena in a new area would be marked by -powerful subterranean disturbances within it, due to the efforts of the -imprisoned vapours to force for themselves a channel to the surface. - -[Sidenote: NATURE OF FIRST EJECTIONS FROM FISSURES.] - -In the case of Monte Nuovo, we have seen that the fissure, when -produced, emitted water--at first in a cold, then in a boiling -condition--and, eventually, steam and scoriæ. It is probable that -through the first cracks which reached the surface, during the heaving -of the subterranean forces, water, charged with carbonic acid, flowed -abundantly, and that these cold springs, charged with carbonic acid -and carbonate of lime, would be succeeded by others which were hot -and contained silica in solution. In Hungary, the Western Isles of -Scotland, and many other volcanic districts, we find abundant evidence -that, before the eruption of lavas in the area, great masses of -travertine and siliceous sinter were formed by the action of cold and -hot springs. - -As the volcanic action became more intense by the more perfect opening -of the fissures, the evolution of carbonic add gas would be succeeded -by the appearance of sulphurous acid, sulphuretted hydrogen, boracic -acid, and hydrochloric acid, which recent studies have shown to be -successively emitted from volcanic vents as the temperature within them -rises. At last lava or molten rock becomes visible within the fissures, -and the ejection of the frothy masses--scoriæ, pumice, lapilli and -dust--commences, and this is sometimes succeeded by the outflow of -currents of lava. - -That volcanoes originate upon lines of fissure in the earth's crust -we have the most convincing proofs. Not only have such fissures been -seen in actual course of formation at Vesuvius, Etna, and other active -volcanoes, but a study of the volcanoes dissected by denudation affords -the most convincing evidence of the same fact. The remarkable linear -arrangement seen in groups of volcanoes, which is conspicuous to the -most superficial observer, confirms this conclusion. - -[Illustration: Fig. 81.--Map of the volcanic group of the Lipari -Islands, illustrating the position of the lines of fissure on which the -volcanoes have been built up.] - -[Sidenote: SHIFTING OF VENTS ALONG FISSURES.] - -We have described the action going on at Stromboli as typical of that -which occurs at all volcanic vents. Stromboli is, however, one among -a group of islands all of which are entirely of volcanic origin. The -volcanoes of this group of islands, the Æolian or Lipari Islands, are -arranged along a series of lines which doubtless mark fissures in the -earth's crust. These fissures, as will be seen by the accompanying map -(fig. 81), radiate from a centre at which we have proofs of the former -existence of a volcano of enormous dimensions. It is a very interesting -fact, which the studies of Prof. Suess have established, that the -earthquakes which have so often desolated Calabria appear to have -originated immediately beneath this great centre of volcanic activity. - -[Illustration: Fig. 82.--The Puy de Pariou in the Auvergne, -illustrating the shifting of the centre of eruption along a line of -fissures.] - -When two volcanic cones are thrown up on the same line of fissure, -their full development is interfered with, and irregularities in their -form and characters are the consequence. In the plan (fig. 82) and the -section (fig. 83) an example is given of the results of such a shifting -of the centre of eruption along a line of fissure. By the second -outburst, one-half of the first-formed cone has been removed, and the -second-formed overlaps the first. - -[Illustration: Fig. 83.--Ideal section of the Puy de Pariou.] - -Sometimes a number of scoria- or tuff-cones are thrown up in such close -proximity to one another along a line of fissure, that they merge into -a long irregular heap on the summit of which a number of distinct -craters can be traced. An example of this kind was furnished by the -line of scoria-cones formed above the fissure which opened on the -flanks of Etna in 1865 (see fig. 84). - -[Illustration: Fig. 84.--Fissure formed on the flanks of Etna during -the eruption of 1865. - -_a._ Monte Frumento, an old parasitic cone. _b._ Line of fissure. _c, -c, c._ New scoria-cones thrown up on line of fissure. _d._ Lava from -same.] - -[Illustration: Fig. 85.--Plan of the Island of Vulcano, based on the -map of the Italian Government.] - -[Sidenote: SHIFTING OF ERUPTIONS ALONG FISSURES.] - -Even in the case of great composite cones, however, we sometimes find -proofs of the centre of eruption having shifted its place along the -line of fissure. No better example of this kind could possibly be -adduced than that of the Island of Vulcano, with the peninsula of -Vulcanello, which is joined to it by a narrow isthmus (see the map, -fig. 81, p 192). In fig. 85 we have given an enlarged plan of this -island which will make its peculiar structure more intelligible (see -also the section given in fig. 77, No. 6, facing p. 178). - -The south-eastern part of the island consists of four crater-rings, -one half of each of Which has been successively destroyed, through -the shifting of the centre of eruption towards the north-west, along -the great line of fissure shown in the general map (fig. 81). The -last formed of these four crater-rings is the one which is now most -complete, and culminates in Monte Saraceno (1581 ft.), _a_ in the -plan, the highest point in the island. The older crater-rings have -been in part removed by the inroads of the waters of the Mediterranean -on the shores of the island. In the centre of the great crater, -_b_, which we have just described, rises the present active cone of -Vulcano, 1,266 feet high, and having a crater, _c_, about 600 yards -in diameter and more than 500 feet in depth. From this cone, a great -stream of obsidian, _e_, flowed in the year 1775, and a small crater, -_d_, the Fossa Anticha, has been opened in the side of the cone. The -continuation of the same line of fissure is indicated by a ruined -tuff-cone, _f_, known as the Faraglione, and the three scoria-cones of -Vulcanello, _g, h_, which have been thrown up so close to one another -as to have their lower portions merged in one common mass, as shown in -fig. 86. - -[Sidenote: SYSTEMS OF VOLCANIC FISSURES.] - -Even in volcanoes of the largest dimensions we sometimes find proofs of -the centre of eruption having shifted along the line of fissure. Lyell -showed that such a change in the position of the central axis of the -volcano had taken place in Etna, and the same phenomenon is exhibited -in the clearest manner' by some of the ancient volcanoes of the Inner -Hebrides, which have been dissected by the denuding forces. - -[Illustration: Fig. 86.--Vulcanello, with its three craters. - -_a._ The most recently-formed and perfect crater, _b_ and _c_. Older -craters, the walls of which have been partly removed by denudation, -_e._ Lava-currents proceeding from _b_. The section exposed in the -cliff at _d_ is represented in fig. 35, p. 116.] - -In the case of the Lipari Islands, the fissures along which the -volcanic mountains have been thrown up radiate from a common centre, -and a similar arrangement can be traced in many volcanic regions, -especially those in which a great central volcano has existed. In other -cases, however, as in the Campi Phlegræi, the volcanic vents appear -to be formed along lines which assume a parallel arrangement, and -this doubtless marks the relative position of the original fissures -produced in the earth's crust when these volcanoes were formed. In some -other cases we find evidences of the existence of a principal fissure -from the sides of which smaller cracks originated. These three kinds of -arrangements of volcano-producing fissures are equally well illustrated -when we study those denuded districts, in which, as we have seen, the -ground-plans of volcanic structures are revealed to our view. - -There is now good ground for believing that in volcanic vents, at which -long-continued eruptive action takes place, the lavas of different -chemical composition make their appearance in something like a definite -order. It had been remarked by Scrope and other geologists at the -beginning of the present century, that in many volcanic areas the acid -or trachytic lavas were erupted before the basic or basaltic. - -Von Richthofen, by his studies in Hungary and the volcanic districts -of the Rocky Mountains, has been able to enunciate a law governing the -natural order of succession of volcanic products; and although some -exception to this law may be mentioned, it is found to hold good for -many other districts than those in which it was first determined. - -In a great number of cases it has been found that the first erupted -rocks in a volcanic district are those of intermediate composition -which are known as andesites. These andesites, which are especially -characterised by the nature of their felspar, sometimes contain free -quartz and are then known as quartz-andesites or dacites, from their -abundance in Transylvania, the old Roman province of Dacia. - -[Sidenote: ORDER OF ERUPTION OF VOLCANIC PRODUCTS.] - -Von Richthofen suggests that another class of volcanic rocks, to which -he gives the name of 'propylites,' were in every case erupted before -the andesites, and in support of his views adduces the fact that in -many instances propylites are found underlying andesites. But the -propylites are, in chemical composition, identical with the andesites, -and like them present some varieties in which quartz occurs, and others -in which that mineral is absent. In their microscopic characters the -propylites differ from the andesites and dacites only in the fact that -the former are more perfectly crystalline in structure, being indeed in -many cases quite undistinguishable from the diorites or the plutonic -representatives of the andesites. The propylites also contain liquid -cavities, which the andesites and dacites as a rule do not, and the -former class of rocks, as Prof. Szabo well points out, are usually much -altered by the passage of sulphurous and other vapours, in consequence -of which they frequently contain valuable metallic ores. - -The extrusion of these andesitic lavas is sometimes accompanied, and -sometimes preceded or followed, by eruptions of trachytic lavas--that -is, of lavas of intermediate composition which have a different kind of -felspar from that prevailing in the andesites. - -In the final stages of the eruptive action in most volcanic districts -the lavas poured forth belong to the classes of the rhyolitic or acid, -and the basaltic or basic lavas. - -These facts are admirably illustrated in the case of the volcanic -district of the Lipari Islands, to which we have had such frequent -occasion to refer. The great central volcano of this district, which -now in a ruined condition constitutes a number of small islets (see -the map, fig. 81, p. 192), is composed of andesitic lavas. The other -great volcanoes thrown up along the three radiating lines of fissure -are composed of andesitic and trachytic rocks. But all the more recent -ejections of the volcanoes of the district have consisted either of -rhyolites, as in Lipari and Vulcano, or of basalts, as in Stromboli and -Vulcanello. - -Von Richthofen and the geologists who most strongly maintain the -generalisations which he has made concerning the order of appearance -of volcanic products, go much farther than we have ventured to do, -and insist that in all volcanic districts a constant and unvarying -succession of different kinds of lavas can be made out. It appears to -us, however, that the exceptions to the law, as thus precisely stated, -are so numerous as to entirely destroy its value. - -The generalisation that in most volcanic districts the first ejected -lavas belong to the intermediate group of the andesites and trachytes, -and that subsequently the acid rhyolites and the basic basalts made -their appearance, is one that appears to admit of no doubt, and is -found to hold good in nearly all the volcanic regions of the globe -which have been attentively studied. - -The Tertiary volcanic rocks of our own country, those of North -Germany, Hungary, the Euganean Hills, the Lipari Islands, and many -other districts in the Old World, together with the widespread volcanic -rocks of the Rocky Mountains in the New World, all seem to conform to -this general rule. - -[Sidenote: THEORY OF VOLCANIC MAGMAS.] - -In connection with this subject, it may be well to refer to the -ideas on the composition of volcanic rocks which were enunciated by -Bunsen, and the theoretic views based on them by Durocher. Bunsen -justly pointed out that all volcanic rocks might be regarded as -mixtures in varying proportions of two typical kinds of materials, -which he named the 'normal trachytic' and the 'normal pyroxenic' -elements respectively. The first of these corresponds very closely in -composition with the acid volcanic rocks or rhyolites, and the second -with the basic volcanic rocks or basalts. Durocher pointed out that -if quantities of these different materials existed in admixture, the -higher specific gravity of the basic element would cause it gradually -to sink to the bottom, while the acid element would rise to the top. -Carrying out this idea still further, he propounded the theory that -beneath the earth's solid crust there exist two magmas, the upper -consisting of light acid materials, the lower of heavy basic ones; and -he supposed that by the varying intensity of the volcanic forces we may -have sometimes one or the other magma erupted and sometimes varying -mixtures of the two. - -The study of volcanic rocks in recent years has not lent much support -to the theoretic views of Durocher concerning the existence of two -universal magmas beneath the earth's crust; and there are not a few -facts which seem quite irreconcilable with such a theory. Thus we -find evidence that in the adjacent volcanic districts of Hungary and -Bohemia, volcanic action was going on during the whole of the latter -part of the Tertiary period. But the products of the contemporaneous -volcanic outbursts in adjacent areas were as different in character -as can well be imagined. The volcanic rocks all over Hungary present -a strong family likeness; the first erupted were trachytes, then -followed andesites and dacites in great abundance, and lastly rhyolites -and basalts containing felspar. But in Bohemia, the lavas poured out -from the volcanoes during the same period were firstly phonolites -and then basalts containing nepheline and leucite. It is scarcely -possible to imagine that such very different classes of lavas could -have been poured out from vents which were in communication with the -same reservoirs of igneous rock, and we are driven to conclude that the -Hungarian and Bohemian volcanoes were supplied from different sources. - -[Sidenote: SEPARATION OF LAVAS IN RESERVOIRS.] - -But the undoubted fact that in so many volcanic regions the eruption of -andesitic and trachytic rocks, which are of intermediate composition, -is followed by the appearance of the differentiated products, rhyolite -and basalt, which are of acid and basic composition respectively, lends -not a little support to the view that under each volcanic district a -reservoir of more or less completely molten rock exists, and that in -these reservoirs various changes take place during the long periods -of igneous activity. During the earlier period of eruption the heavier -and lighter elements of the contents of these subterranean reservoirs -appear to be mingled together; but in the later stages of the volcanic -history of the district, the lighter or acid elements rise to the top, -and the heavier or basic sink to the bottom, and we have separate -eruptions of rhyolite and basalt. We even find some traces of this -action being carried still further. Among the basalts ejected from the -volcanoes of Northern Germany, Bohemia, Styria, Auvergne, and many -other regions, we not unfrequently find rounded masses consisting of -olivine, enstatite, augite, and other heavier constituents of the rock. -These often form the centre of volcanic bombs, and are not improbably -portions of a dense mass which may have sunk to the bottom of the -reservoirs of basaltic materials. - -In consequence of the circumstance that the eruption of lavas of -intermediate composition usually precedes that of other varieties, we -usually find the central and older portions of great volcanoes to be -formed of andesites, trachytes, or phonolites, while the outer and -newer portions of the mass are made up of acid or basic lavas. This is -strikingly exemplified in the great volcanoes of the Auvergne and the -Western Isles of Scotland, in all of which we find that great mountain -masses have, in the first instance, been built up by extrusions of lava -of the intermediate types, and that through this central core fissures -have been opened conveying basic lavas to the surface. From these -fissures great numbers of basaltic lava-streams have issued, greatly -increasing the height and bulk of the volcanic cones and deluging the -country all around. - -The lavas of intermediate composition--the andesites, trachytes, and -phonolites--possess, as we have already seen, but very imperfect -liquidity as they flow from the volcanic vents. Hence we find them -either accumulating in great dome-shaped masses above the vent or -forming lava-streams which are of great bulk and thickness, but do not -flow far from the orifices whence they issue. The more fusible basaltic -lavas, on the other hand, spread out evenly on issuing from a vent, and -sometimes flow to the distance of many miles from it. This difference -in the behaviour of the intermediate and basic lavas is admirably -illustrated in the volcanic districts of the Auvergne and the Western -Isles of Scotland. - -In other cases, like Vesuvius, we find that great volcanic cones of -trachytic tuff have been built up, and that these masses of fragmentary -trachytic materials have been surrounded and enclosed by the ejection, -at a later date, of great outbursts of basaltic lavas. In still -other cases, of which Rocca Monfina in Southern Italy constitutes an -excellent example, we find that a great crater-ring of trachytic tuffs -has been formed in the first instance, and in the midst of this a cone, -composed of more basic materials, has been thrown up. - -[Sidenote: EXCEPTIONS TO THE GENERAL LAW.] - -In all these volcanoes we see the tendency towards the eruption -of intermediate lavas in the first instance, and of basaltic and -acid lavas at a later date. Valuable, however, as are the early -generalisations of Scrope, and the more precise law enunciated by Von -Richthofen concerning the 'natural order of succession of volcanic -products,' we must not forget that there are to be found a considerable -number of exceptions to them. There are some volcanic centres from -which only one kind of lava has been emitted, and this may be either -acid, basic, or intermediate in composition; and on the other hand, -there are districts in which various kinds of lava have been ejected -from the same vents within a short period of time, in such a way as to -defy every attempt to make out anything like a law as to the order of -their appearance. Nevertheless the rules which we have indicated appear -to hold good in so great a number of cases that they are well worthy -of being remembered, and may serve as a basis on which we may reason -concerning the nature of the action going on beneath volcanic vents. - -From the study of the external appearances of volcanic mountains, -combined with investigations of those which have been dissected by -denudation, we are able to picture to our minds the series of actions -by which the great volcanic mountains of the globe have been slowly and -gradually built up. - -In the first instance the eruptions appear to have taken place at -several points along a line of fissure, but gradually all of these -would become choked up except one which became the centre of -habitual eruption. From this opening, ejections, firstly of lavas of -intermediate composition, and afterwards of basic materials, would take -place, until a volcano of considerable dimensions was built up around -it. But at last a point would be reached in the piling up of this -cone, when the volcanic forces below would be inadequate to the work -of raising the liquid lava through the whole length of the continually -upward-growing tube of the volcano. Under these circumstances the -expansive force of the imprisoned steam would find it easier to rend -asunder the sides of the volcanic cone than to force the liquid -material to the summit of the mountain. If these fissures reached the -surface explosive action would take place, in consequence of the escape -of steam from the glowing mass, and scoria-, tuff-, and lava-cones -would be formed above the fissure. In this way, as we have already -pointed out, the numerous 'parasitic cones' which usually abound on -the flanks of the greater volcanic mountains have been formed. The -extrusion of these masses of scoriæ and lava on the flanks of the -mountain tends, not only to increase the bulk of the mass, but to -strengthen and fortify the sides. For by the powerful expansive force -at work below, every weak place in the cone is discovered and a fissure -produced there; but by the extrusion of material at this fissure, and -still more by the consolidation of the lava in the fissure, the weak -place is converted into one of exceptional strength. - -[Sidenote: INTRUSIVE MASSES BENEATH VOLCANOES.] - -As the sides of the cone are thus continually repaired and strengthened -they are rendered more capable of withstanding the heaving forces -acting from below, and these forces can then only find vent for -themselves by again raising the liquefied lava to the central orifice -of the mountain. Many volcanoes, like Etna, exhibit this alternation of -eruptive action from the crater at the summit of the mountain, and from -fissures opened upon its flanks, the former tending to raise the height -of the volcanic pile, the latter to increase its bulk. - -But at last a stage will be reached when the volcanic forces are -no longer able either to raise the lava up the long column of the -central vent on the one hand, or to rend asunder the strongly-built -and well-compacted flanks of the mountain on the other. It is probably -under these conditions, for the most part, that the lavas find their -way between the masses of surrounding strata and force them asunder in -the way that we have already described. - -In the case of the more fluid basaltic lavas, as was pointed out so -long ago by Macculloch, the liquefied materials may find their way -between the strata to enormous distances from the volcanic centre. Such -extended flat sheets of igneous rock retain their parallelism with the -strata among which they are intruded over large areas, and did not -probably produce any marked phenomena at the surface. - -But in the case of less fluid lavas, such as those of intermediate or -acid composition, for example, the effect would be far otherwise. Such -lavas, not flowing readily from the centre of eruption, would tend to -form great bulky lenticular masses between the strata which they forced -asunder, and, in so doing, could not fail to upheave and fissure the -great mountain-mass above. Vast lenticular masses of trachytic rock, -thus evidently forced between strata, have been described by Mr. G. -K. Gilbert, as occurring in the Henry Mountains of Southern Utah, and -by him have been denominated 'laccolites,' or stone-cisterns. Whether -the great basaltic sheets, like those described by Macculloch, and -those more bulky lenticular reservoirs of rock of which Mr. Gilbert -has given us such an admirable account, were in all cases connected -with the surface, may well be a matter for doubt. It is quite possible -that, in some cases, liquefied masses of rocky materials in seeking -to force their way to the surface only succeeded in thus finding a -way for themselves between the strata, and their energy was expended -before the surface was reached and explosive action took place. But it -is an undoubted fact that beneath many of the old volcanoes, of which -the internal structure is now revealed to us by the action of denuding -forces, great intrusive sheets and laccolites abound; and we cannot -doubt that beneath volcanoes now in a state of eruption, or in those -which have but recently become extinct, similar structures must be in -course of formation. - -[Sidenote: EFFECTS OF INTRUSION BENEATH CONES.] - -That great upheaving forces have operated on volcanoes, subsequently -to the accumulation of their materials, we have sufficient evidence in -the Val del Bove of Etna, the Caldera of Palma, the Corral of Madeira, -&c. In all of these cases we find a radial fissure ('barranco') leading -into a great crateral hollow; and these radial fissures are of such -width and depth that their origin can only be referred to a disruptive -force like that which would be exercised by the intrusion of masses of -more or less imperfectly fluid material between the subjacent strata. -These facts, of course, lend no countenance to the views formerly held -by many geologists, both in Germany and France, that the materials -of which volcanoes are built up were deposited in an approximately -horizontal position, and were subsequently blown up like a gigantic -bubble. In Etna, Palma, and Madeira we find abundant proofs that the -mass existed as a great volcanic cone before the production of the -fissures (barrancos), which we have referred to the force exercised -during the intrusion of great igneous masses beneath them. - -But besides the horizontally-disposed intrusive sheets and laccolites, -great, radiating, vertical fissures are produced by the heaving forces -acting beneath those volcanic centres which have been closed up and -'cicatrised' by the exudation from them of subterranean materials. -These vertical intrusions, which we call dykes, like the horizontal -ones, differ in character, according to the nature of the materials -of which they are composed. Dykes of acid and intermediate lava are -usually of considerable width, and do not extend to great distances -from the centres of eruption. Dykes composed of the more-liquid, basic -lavas, on the other hand, may extend to the distance of hundreds of -miles from the central vent. The way in which comparatively narrow, -basaltic dykes are found running in approximately straight lines -for such enormous distances is a very striking fact, and bears the -strongest evidence to the heaving and expanding forces at work at -volcanic centres, during and subsequently to the extrusion of the -igneous products at the surface. - -These basaltic dykes occur in such prodigious numbers around some -volcanic vents, that the whole of the stratified rocks in the immediate -vicinity are broken up by a complete network of them, crossing and -interlacing in the most complicated fashion. Farther away from the -vents, similar dykes are found in smaller numbers, evidently radiating -from the same centre, and sometimes extending to a distance of more -than a hundred miles from it. Nowhere can we find more beautiful -illustrations of such dykes than in the Western Isles of Scotland. When -composed of materials which do not so easily undergo decomposition as -the surrounding rocks, they stand up like vast walls; but when, on the -other hand, they are more readily acted on by atmospheric moisture than -are the rocks which enclose them, they give rise to deep trenches with -vertical sides, which render the country almost impassable. - -[Sidenote: STRUCTURE OF INTRUSIVE MASSES.] - -The lava consolidating in these horizontal intrusions (sheets and -laccolites), and the vertical intrusions (dykes), is usually more -crystalline in structure than the similar materials poured out at the -surface. In the same dyke or sheet, when it is of great width, we -often find every variation--from a glassy material formed by the rapid -cooling of the mass where it is in contact with other rocks, to the -perfectly crystalline or granitic varieties which form the centre of -the intrusion. It is in these dykes and other intrusions that we find -the most convincing evidence of the truth of the conclusions, which -we have enunciated in a former chapter, concerning the dependence of -the structure of an igneous rock upon the conditions under which it -has consolidated. One material is found, under varying conditions, -assuming the characters of obsidian, rhyolite, quartz-felsite, or -granite; another, under the same set of conditions, taking the form of -tachylyte, basalt, dolerite, and gabbro. - -That these great intrusive masses, sheets and dykes, in their passage -between the sedimentary rocks sometimes find places where the overlying -strata are of such thinness or incoherence that the liquefied rocks are -able to force a way for themselves to the surface, we have the clearest -proof. In some dykes we find the rock in their upper portions losing -its compact character and becoming open and scoriaceous, showing that -the pressure had been so far diminished as to allow of the imprisoned -water flashing into steam. - -All round great volcanoes which have become extinct we frequently find -series of small volcanic cones, which have evidently been thrown up -along the lines where the great lava-filled fissures, which we have -been describing, have reached the surface and given rise to explosive -action there. The linear arrangement of these small cones, which are -thrown up in the plains surrounding vast volcanic mountains that have -become extinct, is very striking. The numerous 'puys' of the Auvergne -and adjoining volcanic regions of Central France are for the most part -small scoria- and lava-cones which were thrown up along great lines -of fissure radiating from the immense, central, volcanic mountains of -the district, after they had become extinct. These scoria-cones and -the small lava-streams which flow from them, as was so well shown by -Mr. Scrope, mark the latest efforts of the volcanic forces beneath the -district before they finally sank into complete extinction. In the -Western Isles of Scotland, as I have elsewhere shown, we can study the -formation of these later-formed cones in the plains around extinct -volcanic mountains, with the additional advantage of having revealed -to us, by the action of the denuding forces, their connection with the -great radiating fissures. - -It has been shown that the several stages in the decline of each -volcanic outburst is marked by the appearance at the vent of certain -acid gases. In the same way, after the ejection of solid materials -from a volcanic vent has come to an end, certain gaseous substances -continue to be evolved; and as the temperature at the vents declines, -the nature of the volatile substances emitted from them undergoes a -regular series of changes. - -[Sidenote: ORDER OF EMISSION OF VOLCANIC GASES.] - - -M. Fouqué, by a careful series of analyses of the gases which he -collected at different gaseous vents, or fumaroles as they are called, -in the crater of Vulcano, has been able to define the general relations -which appear to exist between the temperature at a volcanic orifice -and the volatile substances which issue from it. He found that in -fumaroles, in which the temperature exceeded 360° centigrade, and -in which in consequence strips of zinc were fused by the stream of -issuing gas, the analysis of the products showed sulphurous acid and -hydrochloric add to be present in large quantities, and sulphuretted -hydrogen and carbonic acid in much smaller proportions. Around these -excessively heated fumaroles, the lips of which often appear at night -to be red-hot, considerable deposits of sulphide of arsenic, chloride -of iron, chloride of ammonium, boracic acid, and sulphur were taking -place. - -It was found, however, that as the temperature of the vent declined, -the emission of the sulphurous acid and hydrochloric acid diminished, -and the quantity of sulphuretted hydrogen and carbonic acid mingled -with them was proportionately increased. - -In the same way it appears to be a universal rule that when a volcanic -vent sinks into a condition of temporary quiescence or complete -extinction the powerfully acid gases, hydrochloric acid and sulphurous -acid, make their appearance in the first instance, and at a later stage -these are gradually replaced by sulphuretted hydrogen and carbonic acid. - -Of these facts we find a very beautiful illustration in the Campi -Phlegræi near Naples. With the exception of Monte Nuovo, the volcano -which has most recently been in a state of activity in that district -is the Solfatara. From certain apertures in the floor of the crater of -the Solfatara there issue continually watery vapours, sulphurous acid, -sulphuretted hydrogen, hydrochloric acid, and chloride of ammonium. -The action of these substances upon one another, and upon the volcanic -rocks through which they pass, gives rise to the formation of certain -chemical products which, from a very early period, have been collected -on account of their commercial value. The action of these add gases -upon the surrounding rocks is very marked; efflorescent deposits of -various sulphates and chlorides take place in all the crevices and -vesicles of the rock; sulphur and sulphide of arsenic are also formed -in considerable quantities; and the trachytic tuffs, deprived of their -iron-oxide, alkaline earths and alkalies, which are converted into -soluble sulphates and chlorides, are reduced to a white, powdery, -siliceous mass. Many volcanoes, which have sunk into a state of -quiescence or extinction like the Solfatara of Naples, exhibit the same -tendency to give off great quantities of the powerfully-acid gases -which act upon the surrounding rocks, and deprive them of their colour -and consistency. Such volcanoes are said by geologists to have sunk -into the 'solfatara stage.' - -[Sidenote: SOLFATARA-STAGE OF VOLCANOES.] - -At the Lake of Agnano and some other points in the Campi Phlegræi, -however, we find fissures from which the less-powerfully acid gases, -sulphuretted hydrogen and carbonic acid, issue. These gases as they, -are poured forth from the vents are found to be little, if at all, -above the temperature of the atmosphere. Sulphuretted hydrogen is an -inflammable gas, and in the so-called salses and mud-volcanoes, at -which it is ejected in considerable quantities, it not unfrequently -takes fire and bums with a conspicuous flame. Carbonic acid on account -of its great density tends to accumulate in volcanic fissures and -craters rather than to mingle with the surrounding atmosphere. At the -so-called Grotto del Cane, beside the Lago Agnano, it is the custom to -show the presence of this heavy and suffocating gas by thrusting a dog -into it, the poor animal being revived, before life is quite extinct, -by pouring cold water over it. At the Büdos Hegy or 'stinking hill' of -Transylvania, carbonic acid and sulphuretted hydrogen are emitted in -considerable quantities, and it is possible to take a bath of the heavy -gas, the head being kept carefully above the constant level of the -exhalations. - -Although the stories of the ancient Avernian lake, across which no bird -could fly without suffocation, and of the Guevo Upas, or Poison Valley -of Java, which it has been said no living being can cross, may not -improbably be exaggerations of the actual facts, yet there is a basis -of truth in them in the existence of old volcanic fissures and craters -which evolve the poisonous sulphuretted hydrogen and carbonic acid -gases. - -Besides the gases which we have already named, and which are the most -common at and characteristic of volcanic vents, there are some others -which are not unfrequently emitted. First among these we must mention -boracic acid, which, though not a remarkably volatile substance, is -easily carried along in a fine state of division in a current of steam. -At Monte Cerboli and Monte Rotondo in Tuscany, great quantities of -steam jets accompanied by sulphuretted hydrogen and boracic acid issue -from the rocks, and these jets being directed into artificial basins of -water, the boracic acid is condensed and is recovered by evaporation. -We have already noticed that boracic add is evolved with the gases at -Vulcano and other craters; and the part which this substance plays in -volcanic districts is shown by the fact that many of the rocks, filling -old subterranean volcanic reservoirs, are found to be greatly altered -and to have new minerals developed in their midst through the action -upon them of boracic acid. - -Ammonia and various compounds of carbon, nitrogen, and hydrogen are -among the gases evolved from volcanic vents. In some cases these gases -may be produced by the destructive distillation of organic materials -in the sedimentary rocks through which volcanic outbursts take place. -But it is far from impossible that under the conditions of temperature -and pressure which exist at the volcanic foci, direct chemical union -may take place between substances, which at the surface appear to be -perfectly inert in each other's presence. - -When the temperature at volcanic fissures is no longer sufficiently -high to cause water to issue in the condition of vapour or steam, as is -the case at the 'stufas' which we have described, it comes forth in the -liquid state. Water so issuing from old volcanic fissures may vary in -its temperature, from the boiling point downwards. - -[Sidenote: GEYSERS AND HOT-SPRINGS.] - -When the water issues at a temperature little removed from the boiling -point, it is apt to give rise to intermittent springs or geysers, the -eruptions of which exhibit a remarkable analogy with those of ordinary -volcanoes. Geysers may indeed be described as volcanoes in which heated -water, instead of molten rock, is forced out from the vent by the -escaping steam. They occur in great abundance in districts in which the -subterranean action is becoming dormant or extinct, such as Iceland, -the North Island of New Zealand, and the district of the National Park -in the Rocky Mountains. - -Many attempts have been made to explain the exact mechanism by which -the intermittent action of geysers is produced, but it is not at all -probable that any one such explanation will cover all the varied -phenomena exhibited by them. Like volcanic outbursts, geyser eruptions -doubtless originate in the escape of bubbles of steam through a liquid -mass, and this liberation of steam follows any relief of pressure. -In districts where vast masses of lava are slowly cooling down from -a state of incandescence, and surface waters are finding their way -downwards while subterranean waters are finding their way upwards, -there can be no lack of the necessary conditions for such outbursts. -Sometimes the eruptions of geysers take place at short and regular -intervals, at other times they occur at wide and irregular intervals -of time. In some cases the outbursts take place spontaneously, and at -others the action can be hastened by choking up the vent with stones or -earth. - -Other hot springs, like the Strudel of Carlsbad, rise above the surface -in a constant jet, while most of them issue quietly and flow like -ordinary springs. - -Although the violent and paroxysmal outbursts of volcanic mountains -arrest the attention, and powerfully impress us with a sense of the -volcanic activity going on beneath the earth's surface, yet it may -well be doubted whether the quantity of heat, which the earth gets rid -of by their means, at all approaches in amount that which is quietly -dissipated by means of the numerous 'stufas,' gaseous exhalations, and -thermal springs which occur in such abundance all over its surface. -For while the former are intermittent in their action, and powerful -outbursts are interrupted by long periods of rest, the action of the -latter, though feeble, is usually continuous. - -[Sidenote: EFFECTS OF HOT-SPRINGS.] - -Most people may regard the hot spring of Bath as a very slight -manifestation of volcanic activity. This spring issues at a constant -temperature of 49° C, or 120° Fahr. As, however, no less than 180,000 -gallons of water issue daily from this source, we may well understand -how great is the amount of heat of which the earth's crust is relieved -by its agency. It may indeed be doubted whether its action in this way -is not at least equal to that of a considerable volcano which, though -so much more violent, is intermittent in its action. - -Nor are thermal springs by any means ineffective agents in bringing -materials from the interior of the earth's crust and depositing it -at the surface. The Bath spring contains various saline substances, -principally sulphates and chlorides, in solution in its waters. These -are quietly carried by rivers to the sea, and are lost to our view. The -spring has certainly maintained its present condition since the time -of the Romans, and I find that if the solid materials brought from the -interior of the earth during the last 2,000 years had been collected, -they would form a solid cone equal in height to Monte Nuovo. Yet we -usually regard the Campi Phlegræi as a powerfully-active volcanic -district, and the subterranean action in our own country as quite -unworthy of notice. - -When we remember the fact that on the continent of Europe the hot and -saline springs may be numbered by thousands, and that they especially -abound in districts like Hungary, the Auvergne, the Rhine provinces, -and Central Italy, where volcanic action has recently become extinct, -we shall be able to form some slight idea of the work performed -by these agents, not only in relieving the earth's crust of its -superfluous heat, but in transporting materials in a state of solution -from the interior of that crust and depositing them at the surface. The -vast deposits of siliceous sinter and of travertine also bear witness -to the effects produced by hot and mineral springs. - -Nor is the work of these springs confined to the surface. Mr. John -Arthur Phillips has shown that metallic gold and the sulphide of -quicksilver (cinnabar) have been deposited with the silica and other -minerals formed on the sides of a fissure from which hot springs issue -at the surface. There cannot be any doubt that the metallic veins or -lodes, which are the repositories of most of the metals employed in the -arts, have been formed in cracks connected with great volcanic foci, -the transfer of the various sulphides, oxides, and salts which fill -the vein having been effected either by solution, sublimation, or the -action of powerful currents of steam. - -As the igneous activity of the district declines, the temperature -of the issuing gases and waters diminishes with it, until at last -the volcanic forces appear to wholly abandon that region and to be -transferred to another. - -Yet even after all or nearly all indications of the volcanic agencies -cease to make themselves visible at the surface, occasional tremblings -of the earth's crust show that perfect equilibrium has not been -restored below, but that movements are taking place which result in -shocks that are transmitted through the overlying and surrounding -rock-masses as earthquake vibrations. - -[Sidenote: NATURE OF VOLCANIC CYCLES.] - -Such is the cycle of changes which appears to take place at each -district of the earth's surface, as it successively becomes the scene -of volcanic activity. - -The invasion of any particular area of the earth's surface by the -volcanic forces appears to be heralded by subterranean shocks causing -earthquake vibrations. Presently the origination of fissures is -indicated by the rise of saline and thermal springs, and the issuing -of carbonic acid and other gases at the surface. As the subterranean -activity becomes more pronounced, the temperature of the springs and -emitted gases is found to increase, and at last a visible rent is -formed at the surface, exposing the incandescent materials below. - -From this open fissure which has thus been formed, the gas and vapours -imprisoned in the incandescent rock-materials escape with such violence -as to disperse the latter in scoriæ and dust, or to cause them to -well out in great streams as lava-flows. Usually the action becomes -concentrated at one or several points at which the ejected materials -accumulate to form volcanic cones. - -Sometimes the volcanic activity dies away entirely after these cones -are thrown up along the line of fissure, but at others some such -centre becomes for a longer or shorter time the habitual vent for the -volcanic forces in the district, and by repeated ejections of lavas and -fragmentary materials at longer or shorter intervals the cone increases -both in height and bulk. - -When the height of the cone has grown to a certain extent, it becomes -more easy for the volcanic energies below to rend the sides of the -cone than to raise the molten materials to its summit. In this way -lateral or parasitic cones are thrown up on the flanks of the volcanic -mountain, the mass being alternately elevated and strengthened by the -ejections from the summit and sides respectively. - -When the volcanic energies no longer suffice to raise the fluid -materials to the summit, nor to rend the sides of the volcano, fissures -with small cones may be formed in the plains around the great central -volcano. - -At last, however, this energy diminishes so far that rock materials -can no longer be forced to the surface, the fissures become sealed up -by consolidating lava, and the volcanic cones fall into a condition of -extinction and decay. - -The existence of heated materials at no great depth from the surface -is indicated by the outburst of gases and vapours, the formation of -geysers, mud-volcanoes, and ordinary thermal springs. But as the -underlying rocks cool down, the issuing jets of gas and vapour lose -their high temperature and diminish in quantity, the geysers and -mud-volcanoes become extinct, and the thermal springs lose their -peculiar character or disappear, and thus all manifestations of the -igneous energies in the district gradually die away. - -[Sidenote: DURATION OF VOLCANIC CYCLES.] - -Such a cycle of changes probably requires many hundreds of thousands, -or even many millions, of years for its accomplishment; but by the -study of volcanoes in every stage of their growth and decline we are -able to reconstruct even the minutest details of their history. - - - - -CHAPTER VIII. - -THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE OF THE GLOBE. - - -It is not by any means an easy task to frame an estimate of the number -of volcanoes in the world. Volcanoes, as we have seen, vary greatly -in their dimensions--from vast mountain masses, rising to a height -of nearly 25,000 feet above the sea-level, to mere molehills; the -smaller ones being in many cases subsidiary to larger, and constituting -either parasitic cones on their flanks, or 'puys' around their bases. -Volcanoes likewise exhibit every possible stage of development and -decay: while some are in a state of chronic active eruption, others -are reduced to the condition of solfataras, and others again have -fallen into a more or less complete state of ruin through the action of -denuding forces. - -Even if we confine our attention to the larger volcanoes, which merit -the name of 'mountains,' and such of these as we have reason to -believe to be in a still active condition, our difficulties will be -diminished, but not by any means removed. Volcanoes, as we have seen, -may sink into a dormant condition that may endure for hundreds or -even thousands of years, and then burst forth into a state of renewed -activity; and it is quite impossible, in many cases, to distinguish -between the conditions of dormancy and extinction. Concerning certain -small areas in Southern Europe, Western Asia, and Northern Africa, -historical records, more or less reliable, extend back over periods of -several thousands of years; but with regard to the greater part of the -rest of the world we have no information beyond a few hundred years, -and there are considerable areas which have been known only for far -shorter periods, while some are as yet quite unexplored. In districts -almost wholly uninhabited, or roamed over by nomadic tribes, legend -and tradition constitute our only guides--and very unsafe ones they -are--in the attempt to determine what volcanoes have recently been in a -condition of activity. - -[Sidenote: NUMBER OF ACTIVE VOLCANOES.] - -We shall, however, probably be within the limits of truth in stating -that the number of great habitual volcanic vents upon the globe, -which we have reason to believe are still in an active condition, is -somewhere between 300 and 350. Most of these active volcanic vents -are marked by more or less considerable mountains, composed of the -materials ejected from them. If we include the mountains which exhibit -the external conical form, the crateral hollows, and other features -of volcanoes, but concerning the activity of which we have no record -or tradition, the number will fall little, if anything, short of -1,000. The mountains composed of volcanic materials, but which have -lost through denudation the external form of volcanoes, are still more -numerous. The smaller temporary openings which are usually subordinate -to the habitual vents, that have been active during the periods -covered by history and tradition, must be numbered by thousands and -tens of thousands. The still feebler manifestations of the volcanic -forces--such as are exhibited in 'stufas,' or steam-jets, geysers, -or intermittent hot springs, thermal and mineral waters, fumaroles, -emitting various gases, salses or spouting saline and muddy springs, -and mud volcanoes--may be reckoned by millions. It is not improbable -that these less powerful manifestations of the volcanic forces, to a -great extent make up in number what they want in individual energy; and -the relief which they afford to the imprisoned activities within the -earth's crust may be scarcely less than that which results from the -occasional outbursts at the 300 or 350 great habitual volcanic vents. - -In taking a general survey of the volcanic phenomena of the globe, -no fact comes out more strikingly than that of the very unequal -distribution, in different districts, both of the great habitual -volcanic vents, and of the minor exhibitions of subterranean energy. - -[Sidenote: VOLCANOES OF THE CONTINENTS.] - -Thus, on the whole of the continent of Europe, there is but one -habitual volcanic vent--that of Vesuvius--and this is situated upon -the shores of the Mediterranean. In the islands of the Mediterranean, -however, there are no less than six volcanoes; namely, Stromboli and -Vulcano, in the Lipari Islands; Etna, in Sicily; Graham's Isle, a -submarine volcano, off the Sicilian coast; and Santorin and Nisyros, in -the Ægean Sea. - -The African continent is at present known to contain about ten active -volcanoes--four on the west coast, and six on the east coast; about ten -other active volcanoes occur on islands close to the African coasts. In -Asia, twenty-four active volcanoes are known, but no less than twelve -of these are situated in the peninsula of Kamtschatka. No volcanoes are -known to exist in the Australian continent. - -The American continent contains a greater number of volcanoes than -the divisions of the Old World. There are twenty in North America, -twenty-five in Central America, and thirty-seven in South America. - -Thus, taken altogether, there are about one hundred and seventeen -volcanoes situated on the great continental lands of the globe, while -nearly twice as many occur upon the islands scattered over the various -oceans. - -Upon examining further into the distribution of the continental -volcanoes, another very interesting fact presents itself. The volcanoes -are in almost every case situated either close to the coasts of the -continent, or at no great distance from them. There are, indeed, only -two exceptions to this rule. In the great and almost wholly unexplored -table-land lying between Siberia and Tibet four volcanoes are said to -exist, and in the Chinese province of Mantchouria several others. More -reliable information is, however, needed concerning these volcanoes, -situated, unlike all others, at a great distance from the sea. - -It is a remarkable circumstance that all the oceanic islands which -are not coral-reefs are composed of volcanic rocks; and many of these -oceanic islands, as well as others lying near the shores of the -continents, contain active volcanoes. - -Through the midst of the Atlantic Ocean runs a ridge, which, by the -soundings of the various exploring vessels sent out in recent years, -has been shown to divide the ocean longitudinally into two basins. -Upon this great ridge, and the spurs proceeding from it, rise numerous -mountainous masses, which constitute the well-known Atlantic islands -and groups of islands. All of these are of volcanic origin, and among -them are numerous active volcanoes. The Island of Jan Mayen contains -an active volcano, while Iceland contains thirteen, and not improbably -more; the Azores have six active volcanoes, the Canaries three; while -about eight volcanoes lie off the west coast of Africa. In the West -Indies there are six active volcanoes; and three submarine volcanoes -have been recorded within the limits of the Atlantic Ocean. Altogether, -no less than forty active volcanoes are situated upon the great -submarine ridges which traverse the Atlantic longitudinally. - -[Sidenote: VOLCANOES ON THE OCEANIC ISLANDS.] - -But along the same line the number of extinct volcanoes is far greater, -and there are not wanting proofs that the volcanoes which are still -active are approaching the condition of extinction. At a somewhat -earlier period of the earth's history the whole line of the present -Atlantic Ocean was in all probability traversed by a chain of volcanoes -on the very grandest scale; but submergence has taken place, and only a -few portions of this great mountain range now rise above the sea-level, -forming the isolated islands and island-groups of the Atlantic. Here -and there among these a still active volcano exists. - -But if the great medial chain of the Atlantic presents us with an -example of a chain of volcanic mountains verging on extinction, we have -in the line of islands separating the Pacific and Indian Oceans an -example of a similar range of volcanic vents which are in a condition -of the greatest activity. In the peninsula of Kamtschatka there are -twelve active volcanoes, in the Aleutian Islands thirty-one, and in -the peninsula of Alaska three. The chain of the Kuriles contains at -least ten active volcanoes; the Japanese Islands and the islands lying -to the south of Japan twenty-five. The great group of islands lying -to the south-east of the Asiatic continent is at the present time the -grandest focus of volcanic activity upon the globe. No less than fifty -active volcanoes occur here. Farther south, the same chain is probably -continued by the four active volcanoes of New Guinea, one or more -submarine volcanoes, and several vents in New Britain, the Solomon -Isles, and the New Hebrides, the three active volcanoes of New Zealand, -and possibly by Mount Erebus and Mount Terror in the Antarctic region. -Altogether, no less than 150 active volcanoes exist in the chain of -islands which stretch from Behring's Straits down to the Antarctic -circle; and if we include the volcanoes on Indian and Pacific islands -which appear to be situated on lines branching from this particular -band, we shall not be wrong in the assertion that this great system of -volcanic mountains includes at least one half of the habitually active -vents of the globe. - -A third series of volcanoes starts from near the last in the -neighbourhood of Behring's Straits, and stretches along the whole -western coast of the American continent. In this great range there are -about eighty active volcanoes. - -[Sidenote: LINEAR ARRANGEMENT OF VOLCANOES.] - -In considering the facts connected with the distribution of volcanoes -upon the globe, the one which, by its striking character, seems to -demand our attention in the first instance is that of the remarkable -linear arrangement of volcanic vents. We have already seen that small -scoria-cones are often thrown up on the flanks, or at the base, of a -great volcanic mountain, along lines which are manifestly lines of -fissure. In the eruption of Etna, in 1865, and again in that of 1874, -Professor Silvestri, of Catania, witnessed the actual opening of great -fissures on the north-east and north sides of the mountain: and -along the bottom of these cracks the glowing lava was clearly visible -(fig. 84, page 194). In the course of a few days, there were thrown -up a number of small scoria-cones along these lines of fissure--those -formed on the fissure of 1865 being seven in number, and those on the -fissure of 1874 being no less than thirty-six in number. Precisely -familiar phenomena were witnessed upon the slopes of Vesuvius, in 1760, -when a fissure opened on the south side of the mountain, and fifteen -scoria-cones, which are still visible, were thrown up along it. - -We have already considered the evidence pointing to the conclusion that -systems of volcanoes, like that of the Lipari Islands, are similarly -ranged along lines of fissures, and there is equally good ground for -believing that the great linear bands of volcanoes, which, as we -have seen, stretch for thousands of miles, have had their positions -determined by great lines of fissure in the earth's crust. While, -however, the smaller fissures, upon which rows of scoria-cones are -thrown up, seem to have been in many cases opened by a single effort -of the volcanic forces, the enormous fissures, which traverse so large -a portion of the surface of the globe, are doubtless the result of -numerous manifestations of energy extending over vast periods of time. - -The greatest of these bands along which the volcanic forces are so -powerfully exhibited at the present day, is the one which stretches -from near the Arctic circle at Behring's Straits to the Antarctic -circle at South Victoria. The line followed by this volcanic band, -which, as we have seen, includes more than one half of the active -volcanoes of the globe, is a very sinuous one, and it gives off -numerous offshoots upon either side of it. The great focus of this -intense volcanic action may be regarded as lying in the district -between the islands of Borneo and New Guinea. From this centre there -radiate a number of great lines, along which the volcanic forces are -exhibited in the most powerful manner. The first of these extends -northwards through the Philippine Isles, Japan, the Kurile Islands, and -Kamtschatka, giving off a branch to the east, which passes through the -Aleutian Islands and the peninsula of Alaska. This band, along which -the volcanic forces are very powerfully active, is continued towards -the south-east in the New Britain, the Solomon Islands, Santa Cruz, the -New Hebrides, New Zealand, and South Victoria. East and west from the -great central focus there proceed two principal branches. The former -of these extends through the Navigator Islands and Friendly Islands -as far as Elizabeth Islands. The latter passes through Java, and then -turns north-westward through Sumatra, the Nicobar Islands, the Andaman -Islands up to the coast of Burmah. - -The great band which we have been describing exhibits the most -striking examples of volcanic activity to be found upon the globe. -Besides the 150 or more volcanoes which are known to have been in a -state of activity during the historical period, there are several -hundred very perfect volcanic cones, many of which appear to have but -recently become extinct, if indeed, they are not simply in a dormant -condition. For long distances these chains of volcanic cones are almost -continuous, and the only very considerable breaks in the series are -those between New Zealand and the New Hebrides on the one hand, and -between the former islands and South Victoria on the other. - -[Sidenote: GREAT VOLCANIC BANDS OF THE GLOBE.] - -Much less continuous, but nevertheless very important, is the great -band of volcanoes which extends along the western side of the great -American continent, and contains, with its branches, nearly a hundred -active volcanoes. On the north this great band is almost united with -the one we have already described by the chain of the Aleutian and -Alaska volcanoes. In British Columbia about the parallel of 60° N. -there exist a number of volcanic mountains, one of which, Mount St. -Elias, is believed to be 18,000 feet in height, and several of these -have certainly been seen in a state of eruption. Farther south in the -part of the United States, territories drained by the Columbia River, -a number of grand volcanic mountains exist, some of which are probably -still active, for geysers and other manifestations of volcanic activity -abound. From the southern extremity of the peninsula of California -an almost continuous chain of volcanoes stretches through Mexico and -Guatemala, and from this part of the volcanic band a branch is given -off which passes through the West Indies, and forms a connection with -the great volcanic band of the Atlantic Ocean. In South America the -line is continued by the active volcanoes of Ecuador, Bolivia and -Chili, but at many intermediate points in the chain of the Andes -extinct volcanoes occur, which to a great extent fill up the gaps -in the series. A small offshoot to the westward passes through the -Galapagos Islands. The great band of volcanoes which stretches through -the American continent is second only in importance, and in the -activity of its vents, to the band which divides the Pacific from the -Indian Ocean. - -The third volcanic band of the globe is that which traverses the -Atlantic Ocean from north to south. This series of volcanic mountains -is much more broken and interrupted than the other two, and a greater -proportion of its vents are extinct. This chain, as we shall show in a -future chapter, attained its condition of maximum activity during the -distant period of the Miocene, and now appears to be passing into a -state of gradual extinction. Beginning in the north with the volcanic -rocks of Greenland and Bear Island, we pass southwards, by way of Jan -Mayen, Iceland, and the Faroe Islands, to the Hebrides and the north -of Ireland. Thence by way of the Azores, the Canaries and the Cape de -Verde Islands, with some active vents, we pass to the ruined volcanoes -of St. Paul, Fernando de Noronha, Ascension, St. Helena, Trinidad and -Tristan d'Acunha. From this great Atlantic band two branches proceed -to the eastward, one through Central Europe, where all the vents are -now extinct, and the other through the Mediterranean to Asia Minor, -the great majority of the volcanoes along the latter line being now -extinct, though a few are still active. The vol canoes on the eastern -coast of Africa may be regarded as situated on another branch from this -Atlantic volcanic band. The number of active volcanoes on this Atlantic -band and its branches, exclusive of those in the West Indies, does not -exceed fifty. - -[Sidenote: LENGTH OF THE VOLCANIC BANDS.] - -From what has been said, it will be seen that, not only do the -volcanoes of the globe usually assume a linear arrangement, but nearly -the whole of them can be shown to be thrown up along three well-marked -bands and the branches proceeding from them. The first and most -important of these bands is nearly 10,000 miles in length, and with its -branches contains more than 150 active volcanoes; the second is 8,000 -miles in length, and includes about 100 active volcanoes; the third is -much more broken and interrupted, extends to a length of nearly 1,000 -miles, and contains about 50 active vents. The volcanoes of the eastern -coast of Africa, with Mauritius, Bourbon, Rodriguez, and the vents -along the line of the Red Sea, may be regarded as forming a fourth and -subordinate band. - -Thus we see that the surface of the globe is covered by a network -of volcanic bands, all of which traverse it in sinuous lines with a -general north-and-south direction, giving off branches which often -run for hundreds of miles, and sometimes appear to form a connection -between the great bands. - -These four bands of volcanic vents, running in a general -north-and-south direction, separate four unequal areas within which the -exhibitions of volcanic activity are feeble or quite unknown. The two -grandest of the bands of volcanic activity, with their branches, form -an almost complete series encircling the largest of the oceans. - -To this rule of the linear arrangement of the volcanic vents of the -globe and their accumulation along certain well-marked bands, there are -two very striking exceptions, which we must now proceed to notice. - -In the very centre of the continent formed by Europe and Asia, the -largest unbroken land-mass of the globe, there rises from the great -central plateau the remarkable volcanoes of the Thian Shan Range. -The existence of these volcanoes, of which only obscure traditional -accounts had reached Europe before the year 1858, appears to be -completely established by the researches of the Russian traveller -Semenof. Three volcanic vents appear to exist in this region: the -active volcanoes of Boschan and Turfan or Hot-schen, and the solfatara -of Urumtsi. At a point situated about half-way between these three -volcanoes and the sea, another active vent, that of Ujung-Holdongi, is -said to exist. Other volcanic phenomena have been stated to occur in -the great plateau of Central Asia, but the existence of some at least -of these appears to rest on very doubtful evidence. The only accounts -which we have of the eruptions of these Thian Shan volcanoes are -contained in Chinese histories and treatises on geography; and a great -service would be rendered to science could they be visited by some -competent explorer. - -[Sidenote: EXCEPTIONALLY-SITUATED VOLCANOES.] - -The second exceptionally-situated volcanic group is that of the -Sandwich Islands. While the Thian Shan volcanoes rise in the centre of -the largest unbroken land-mass, and stand on the edge of the loftiest -and greatest plateau in the world, the volcanoes of the Sandwich -Islands rise almost in the centre of the largest ocean and from almost -the greatest depths in that ocean. All round the Sandwich Islands the -sea has a depth of from 2,000 to 3,000 fathoms, and the island-group -culminates in several volcanic cones which rise to the height of nearly -14,000 feet above the sea-level. The volcanoes of the Sandwich Islands -are unsurpassed in height and bulk by those of any other part of the -globe. - -With the exception of the two isolated groups of the Thian Shan and -the Sandwich Islands, nearly all the active volcanoes of the globe -are situated near the limits which separate the great land- and -water-masses of the globe--that is to say, they occur either on the -parts of continents not far removed from their coast-lines, or on -islands in the ocean not very distant from the shores. - -The fact of the general proximity of volcanoes to the sea, is one which -has frequently been pointed out by geographers, and may now be regarded -as being thoroughly established. Even the apparently anomalous case of -the Thian Shan volcanoes is susceptible of explanation if we remember -the fact, now well ascertained by geological researches, that as -late certainly as Pliocene times, a great inland sea spread over the -districts where the Caspian, the Sea of Aral, and many other isolated -lakes are now found. Upon the southern shore of this sea rose the -volcanoes of the Thian Shan, some of which have not yet fallen into a -state of complete extinction. - -But although the facts concerning the general proximity of volcanoes to -the ocean may be admitted to be thoroughly established, yet inferences -are sometimes hastily drawn from these facts which the latter, if -fairly considered, will not be found to warrant. It is frequently -assumed that we may refer all the remarkable phenomena of volcanic -action to the penetration of sea-water to a mass of incandescent lava -in the earth's crust, and to the chemical or mechanical action which -would result from this meeting of sea-water and molten rock. And this -conclusion is supposed to find support in the circumstance that many -of the gases and volatile substances emitted from volcanic vents are -such as would be produced by the decomposition of the various salts -contained in sea-water. - -This argument in favour of the production of volcanic outbursts by -the irruption of sea-water into subterranean reservoirs, involves, as -Mr. Scrope long ago pointed out, a curious example of reasoning in a -circle. It is assumed, on the one hand, that the heaving subterranean -movements, which give rise to the fissures by which steam and other -gases escape to the surface, are the result of the passage of water -to heated masses in the earth's crust. But, on the other hand, it is -supposed that it is the production of these fissures which leads to -the influx of water to the heated materials. If it is the passage of -water through these fissures which produces the eruptions, it may be -fairly asked, what is it that gives rise to the fissures? And if, on -the other hand, there exist subterranean forces competent to produce -the fissures, may they not also give rise to the eruptions through the -openings which they have originated? Nor does the chemical argument -appear to rest upon any surer ground. It is true that many of the -volatile substances emitted from volcanic vents are such as might be -produced by the decomposition of sea-water, but, upon the other hand, -there are not a few substances which cannot possibly be regarded as so -produced, and, all the materials may equally well be supposed to have -been originally imprisoned in the masses of subterranean lava. - -[Sidenote: CAUSE OF PROXIMITY OF VOLCANOES TO SEA.] - -The problem before us is this. Granting that it is proved that active -volcanoes are always in close proximity to the ocean, are we to explain -the fact by supposing that the agency of sea-water is necessary to -volcanic outbursts, or by regarding the position of the coast-lines -as to some extent determined by the distribution of volcanic action -upon the surface of the globe? The first supposition is the one which -perhaps most readily suggests itself, but the latter, as we shall -hereafter show, is one in favour of which not a few weighty arguments -may be advanced. - -Another problem which suggests itself in connection with the -distribution of volcanoes is the following. Are the great depressed -tracts which form the bottom of the oceans, like the elevated tracts -which constitute the continents, equally free from exhibitions of -volcanic energy? - -When we remember the fact that the area of the ocean beds is two and -three-quarter times as great as that of the continents, it will be seen -how important this question of the existence of volcanoes at the bottom -of the ocean really is. - -The fact that recent deep-sea soundings have shown the deepest parts -of the ocean to be everywhere covered with volcanic _débris_ is by -no means conclusive upon this question; for, as we have seen, the -ejections of sub-aerial volcanoes are by the wind and waves distributed -over every part of the earth's surface. - -[Sidenote: SUBMARINE ERUPTIONS.] - -Submarine volcanic outbursts have occurred in many parts of the -globe, but it may well be doubted whether any such outburst has ever -commenced at the bottom of a deep ocean, and has succeeded in building -up a volcanic cone reaching to the surface. Most, if not all, of the -recorded submarine outbursts have occurred in the midst of volcanic -districts, and the volcanic cones have been built up in water of no -great depth. Indeed, when it is remembered that the pressure of each -1,000 fathoms of water is equivalent to a weight of more than one ton -on every square inch of the ocean-bottom, it is difficult to imagine -the ordinary explosive action of volcanic vents taking place at abysmal -depths. If, however, fissures were opened in the beds of the ocean, -quiet outwellings of lava might possibly occur. - -The solution of this problem of the probable existence of volcanic -outbursts on the floor of the ocean can only be hoped for from the -researches of the geologist. The small specimens of the ocean-beds -brought up by deep-sea sounding-lines, taken at wide distances apart, -and including but a few inches from the surface, can certainly afford -but little information upon the question. But the geologist has the -opportunity of studying the sea-bottoms of various geological periods -which have been upheaved and are now exposed to his view. It was at -one time supposed by geologists that in the so-called 'trap-rocks' we -have great lava-sheets which must have been piled upon one another, -without explosive action. But the more accurate researches of recent -years have shown that between the layers of 'trap-rock,' in every part -of the globe, traces of terrestrial surfaces and freshwater deposits -are found; and the supposed proofs of the absence of explosive action -break down no less signally upon re-examination; for the loose, -scoriaceous materials would either be removed by denudation, or -converted into hard and solid rocks by the infilling of their vesicles -and air-cavities with crystalline minerals. It is not possible, among -the representatives of former geological periods, to point to any rocks -that can be fairly regarded as having issued from great submarine -fissures, and it is therefore fair to conclude that no such great -outbursts of the volcanic forces take plane at the present day on the -deep ocean-floors. - -In connection with the question of the relation between the position of -the volcanic bands of the globe and the areas covered by the ocean, we -may mention a fact which deep-sea soundings appear to indicate, namely, -that the deepest holes in the ocean-floor are situated in volcanic -areas. Near Japan, the soundings of the U.S. ship 'Tuscarora' showed -that at two points the depth exceeded 4,000 fathoms; and the deepest -sounding obtained by H.M.S. 'Challenger,' amounting to 4,575 fathoms, -was taken in the voyage from New Gruinea to Japan, in the neighbourhood -of the Ladrone Islands. Depths nearly as great were found in the -soundings carried on in the neighbourhood of the volcanic group of the -West Indian Islands. It must be remembered, however, that at present -our knowledge of the depths of the abysmal portions of the ocean is -very limited. A few lines of soundings, often taken at great distances -apart, are all we have to guide us to any conclusions concerning the -floors of the great oceans, and between these lines are enormous areas -which still remain altogether unexplored. It may be wise, therefore, to -suspend our judgment upon such questions till more numerous facts have -been obtained. - -[Sidenote: RELATIONS TO MOUNTAIN-CHAINS.] - -Another fact concerning the distribution of volcanoes which is worthy -of remark is their relation to the great mountain-ranges of the globe. - -Many of the grandest mountain-chains have bands of volcanoes -lying parallel to them. This is stinkingly exhibited by the great -mountain-masses which lie on the western side of the American -continent. The Rocky Mountains and the Andes consist of folded and -crumpled masses of altered strata which, by the action of denuding -forces, have been carved into series of ridges and summits. At many -points, however, along the sides of these great chains, we find that -fissures have been opened and lines of volcanoes formed, from which -enormous quantities of lava have flowed and covered great tracts of -country. At some parts of the chain, however, the volcanoes are of such -height and dimensions as to overlook and dwarf the mountain-ranges by -the side of which they lie. Some of the volcanoes lying parallel to the -great American axis appear to be quite extinct, while others are in -full activity. - -In the Eastern continent we find still more striking examples of -the parallelism between great mountain-chains and the lands along -which volcanic activity is exhibited. Stretching in a more or less -continuous chain from east to west, through Europe and Asia, we find -the mountain-masses known in different parts of their course as the -Pyrenees, the Alps, the Balkan, the Caucasus, which form the axis -of the Eastern continent. These chains consist of numerous parallel -ridges, and give off branches on either side of them. They are -continued to the eastward by the Hindoo Koosh and the Himalaya, with -the four parallel ranges that cross the great Central-Asian plateau. -Now, on either side of this grand axial system of mountains, we find -a great parallel band of volcanoes. The northern volcanic band is -constituted by the eruptive rocks of the Auvergne, the Eifel, the -Siebengebirge, Central Germany, Bohemia, Hungary, and Transylvania, -few, if any, of the vents along this northern band being still active. -The remarkable volcanoes of the Thian Shan range and of Mantchouria may -not improbably be regarded as a continuation of the same great series. - -The southern band of volcanoes, lying parallel to the great mountain -axis of the Old World, also consists for the most part of extinct -volcanoes, but includes not a few vents which are still active. In -this band we include the extinct volcanoes of Spain and Sardinia, the -numerous extinct and active vents of the Italian peninsula and islands, -and those of the Ægean Sea and Asia Minor. We may, perhaps, consider -the scattered volcanoes of Arabia and the northern part of the Indian -Ocean as a continuation of the same series. Both of these bands may -be regarded as offshoots from the great mid-Atlantic volcanic chain, -and the condition of the vents, both in the principal band and its -offshoots, is such as to indicate that they form parts of a system -which is gradually sinking into a state of complete extinction. - -There are some other volcanic bands which exhibit a similar parallelism -with mountain chains; but, on the other hand, there are some volcanoes -between which and the nearest mountain axes no such connection can be -traced. - -[Sidenote: RELATION TO AREAS OF UPHEAVAL.] - -There is yet one other fact concerning the mode of distribution of -volcanoes upon the surface of the globe, to which we must allude. It -was first established by Mr. Darwin as one of the conclusions derived -from the valuable series of observations made by him during the voyage -of H.M.S. 'Beagle,' and relates to the position of active volcanoes -with respect to the portions of the earth's crust which are undergoing -upheaval or subsidence. - -From the relative position of the different kinds of coral-reefs, and -the fact that reef-forming corals cannot live at a depth of more than -twenty fathoms beneath the sea-level, or above tide-mark, we are led to -the conclusion that certain areas of the earth's surface are undergoing -slow elevation, while other parts are as gradually subsiding. This -conclusion is confirmed by the occurrence of raised beaches, which -are sometimes found at heights of hundreds, or even thousands, of -feet above the sea-level, and of submerged forests, which are not -unfrequently found beneath the waters of the ocean. - -By a study of the evidences presented by coral-reefs, raised beaches, -submerged forests, and other phenomena of a similar kind, it can be -shown that certain wide areas of the land and of the ocean-floor are -at the present time in a state of subsidence, while other equally -large areas are being upheaved. And the observations of the geologist -prove that similar upward and downward movements of portions of the -earth's crust have been going on through all geological times. Now, -as Mr. Darwin has so well shown in his work on 'Coral-Reefs,' if we -trace upon a map the areas of the earth's surface which are undergoing -upheaval and subsidence respectively, we shall find that nearly all the -active volcanoes of the globe are situated upon rising areas, and that -volcanic phenomena are conspicuously absent from those parts of the -earth's crust which can be proved at the present day to be undergoing -depression. - - - - -CHAPTER IX. - -VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY. - - -It is only in comparatively recent times that the important doctrine of -geological continuity has come to be generally accepted, as furnishing -us with a complete and satisfactory explanation of the mode of origin -of the features of our globe. The great forces, which are ever at -work producing modifications in those features, operate so silently -and slowly, though withal so surely, that without the closest and -most attentive observation their effects may be easily overlooked; -while, on the other hand, there are so many phenomena upon our globe -which seem at first sight to bear testimony to the action of sudden -and catastrophic forces, very different to any which appear to be at -present at work, that the tendency to account for all past changes by -these violent actions is a very strong one. In spite of this tendency, -however, the real potency of the forces now at work upon the earth's -crust has gradually made its way to recognition, and the capability of -these forces, when their effects are accumulated through sufficiently -long periods of time, to bring about the grandest changes, is now -almost universally admitted. The modern science of geology is based -upon the principle that the history of the formation and development -of the earth's surface-features, and of the organisms upon it, has -been continuous during enormous periods of time, and that in the study -of the operations taking place upon the earth at the present day, we -may find the true key to the changes which have occurred during former -periods. - -In no branch of geological science has the doctrine of continuity -had to encounter so much opposition and misconception as in that -which relates to the volcanic phenomena of the globe. For a long time -students of rocks utterly failed to recognise any relation between the -materials which have been ejected from active volcanic vents and those -which have been formed by similar agencies at earlier periods of the -earth's history. And what was far worse, the subject became removed -from the sphere of practical scientific inquiry to that of theological -controversy, those who maintained the volcanic origin of some of the -older rocks being branded as the worst of heretics. - -[Sidenote: CONTROVERSY CONCERNING ORIGIN OF BASALT.] - -With the theological aspects of the great controversy concerning the -origin of basalt and similar rocks--a controversy which was carried -on with such violence and acrimony during the latter half of the -eighteenth century--we have here nothing to do. But it may not be -uninstructive to notice the causes of the strange misconceptions which -for so long a period stood in the way of the acceptance of rational -views upon the subject. - -At this period but little had been done in studying the chemical -characters of aqueous and igneous rock-masses respectively; and while, -on the one hand, the close similarity in chemical composition between -the ancient basalts and many modern lavas was not recognised, the -marked distinction between the composition of such materials and most -aqueous sediments remained, on the other hand, equally unknown. Nor -had anything been yet accomplished in the direction of the study of -rock-masses by the aid of the microscope. Hence there could be no -appeal to those numerous structural peculiarities that at once enable -us to distinguish the most crystalline aqueous rocks from the materials -of igneous origin. - -On the other hand, there undoubtedly exist rocks of a black colour -and crystalline structure, sometimes presenting a striking similarity -in general appearance to the basalts, which contain fossils and are -undoubtedly of aqueous origin. Thus on the shore near Portrush, in the -North of Ireland, and in the skerries which lie off that coast, there -occur great rock-masses, some of which undoubtedly agree with basalt in -all their characters, while others are dark-coloured and crystalline, -and are frequently crowded with _Ammonites_ and other fossils. We now -know that the explanation of these facts is as follows. Near where -the town of Portrush is now situated, a volcanic vent was opened in -Miocene times through rocks of Lias shale. From this igneous centre, -sheets and dykes of basaltic lava were given off, and in consequence -of their contact with these masses of lava, the Lias shales were baked -and altered, and assumed a crystalline character, though the traces of -the fossils contained in them were not altogether obliterated. In the -last century the methods which had been devised for the discrimination -of rocks were so imperfect that no distinction was recognised between -the true basalt and the altered shale, and specimens of the latter -containing _Ammonites_ found their way to almost every museum in -Europe, and were used as illustrations of the 'origin of basalt by -aqueous precipitation.' - -Another source of the widely-spread error which prevailed concerning -the origin of basalt, was the failure to recognise the nature of -the alterations which take place in the character of rock-masses in -consequence of the passage through them, during enormous periods of -time, of water containing carbonic acid and other active chemical -agents. The casual observer does not recognise the resemblance which -exists between certain ornamental marbles and the loose accumulations -of shells and corals which form many sea-beaches; but close examination -shows that the former consist of the same materials as the latter, -bound together by a crystalline infilling of carbonate of lime, which -has been deposited in all the cavities and interstices of the mass. In -the same way, as we have already seen, the vesicles and interstices -of heaps of scoriæ may, by the percolation of water through the mass, -become so filled with various crystalline substances, that its original -characters are entirely masked. - -But the progress of chemical and microscopic research has effectually -removed these sources of error. Many rocks of aqueous origin, formerly -confounded with the basalts, have now been relegated to their proper -places among the different classes of rocks; while, on the other -hand, it has been shown that the chemical and physical differences -between the ancient basalts and the modern basic lavas are slight -and accidental, and their resemblances are of the closest and most -fundamental character. - -[Sidenote: VOLCANIC ORIGIN OF 'TRAP ROCKS.'] - -The notion of the aqueous origin of basalt, which was so long -maintained by the school of Werner, has now been entirely abandoned, -and the so-called 'trap-rocks' are at the present day recognised as -being as truly volcanic in their origin as the lavas of Etna and -Vesuvius. - -There is, however, a vestige of this doctrine of Werner, which still -maintains its ground with obstinate persistence. Many geologists in -Germany who admit that volcanic phenomena, similar to those which are -going on at the present day, must have occurred during the Tertiary and -the later Secondary periods, nevertheless insist that among the earlier -records of the world's history we find no evidence whatever of such -volcanic action having taken place. By the geologists who hold these -views it is asserted that while the granites and other plutonic rocks -were formed during the earlier periods of the world's history, true -volcanic products are only known in connection with the sediment of the -later geological periods. - -Some geologists have gone farther even than this, and asserted that -each of the great geological periods is characterised by the nature -of the igneous ejections which have taken place in it. They declare -that granite was formed only during the earliest geological periods, -and that at later dates the gabbros, diabases, porphyries, dolerites -and basalts, successively made their appearance, and finally that the -modern lavas were poured out. - -A little consideration will suffice to convince us that these -conclusions are not based upon any good evidence. The plutonic rocks, -as we have already seen, exhibit sufficient proofs in their highly -crystalline character, and in their cavities containing water, -liquefied carbonic acid, and other volatile substances, that they must -have been formed by the very slow consolidation of igneous materials -under enormous pressure. Such pressures, it is evident, could only -exist at great depths beneath the earth's surface. Mr. Sorby and -others have endeavoured to calculate what was the actual thickness of -rock under which certain granites must have been formed, by measuring -the amount of contraction in the liquids which have been imprisoned -in the crystals of these rocks. The conclusions arrived at are of a -sufficiently startling character. It is inferred that the granites -which have been thus examined must have consolidated at depths varying -from 30,000 to 80,000 feet beneath the earth's surface. It is true -that in arriving at these results certain assumptions have to be made, -and to these exception may be taken, but the general conclusion that -granitic rocks could only have been formed under such high pressures as -exist at great depths beneath the surface, appears to be one which is -not open to reasonable doubt. - -If, then, granites and similar rocks were formed at the depth of some -miles, it is evident that they can only have made their appearance at -the surface by the removal of the vast thickness of overlying rocks; -and the sole agency which we know of that is capable of effecting the -removal of such enormous quantities of rock-materials, is denudation. -But the agents of denudation--rain and frost, rivers and glaciers, and -sea-waves--though producing grand results, yet work exceeding slowly; -and almost inconceivably long periods of time must have elapsed before -masses of rock several miles in thickness could have been removed, and -the subjacent granites and other highly crystalline rocks have been -exposed at the surface. - -[Sidenote: ANCIENT AND MODERN VOLCANIC ROCKS.] - -It is an admitted fact that among the older geological formations, we -much more frequently find intrusions of granitic rocks than in the case -of younger ones. It is equally true that among the sediments formed -during the most recent geological periods, no true granitic rocks have -been detected. But if, as we insist is the case, granitic rocks can -only be formed at a great depth from the surface, the £acts we have -described are only just what we might expect to present themselves -under the circumstances. The older a mass of granitic rock, the greater -chance there is that the denuding forces operating upon the overlying -masses, will have had an opportunity of so far removing the latter -as to expose the underlying crystalline rocks at the surface. And, -on the other hand, the younger crystalline rocks are still, for the -most part, buried under such enormous thicknesses of superincumbent -materials that it is hopeless for us to search for them. Nevertheless, -it does occasionally happen that, where the work of denudation has been -exceptionally rapid in its action, such crystalline rocks formed during -a comparatively recent geological period, are exposed at the surface. -This is the case in the Western Isles of Scotland and in the Pyrenees, -where masses of granite and other highly crystalline rocks are found -which were evidently formed during the Tertiary period. - -The granites which were formed in Tertiary times present no essential -points of difference from those which had their origin during the -earlier periods of the earth's history. The former, like the latter, -consist of a mass of crystals with no imperfectly crystalline base or -groundmass between them; and these crystals include numerous cavities -containing liquids. - -Between the granites and the quartz-felsites every possible -gradation may be found, so that it is impossible to say where the one -group ends and the other begins; indeed, many of the rocks called -'granite-porphyries' have about equal claims to be placed in either -class. Nor is the distinction between the quartz-felsites and rhyolites -any more strongly marked than that between the former class of rocks -and the granites; some of the more crystalline rhyolites of Hungary -being quite undistinguishable, in their chemical composition, their -mineralogical constitution, and their microscopic characters, from -the quartz-felsites. The more crystalline rhyolites are in turn found -passing by insensible gradations into the glassy varieties and finally -into obsidian. - -[Sidenote: RELATIONS BETWEEN GRANITE AND PUMICE.] - -A piece of granite and a piece of pumice may at first sight appear to -present so many points of difference, that it would seem quite futile -to attempt to discover any connection between them. Yet, if we analyse -the two substances, we may find that in ultimate chemical composition -they are absolutely identical. There is nothing irrational, therefore, -in the conclusion that the same materials under different conditions -may assume either the characters of granite on the one hand, or of -pumice on the other; the former being consolidated under circumstances -in which the chemical and crystalline forces have had the freest play -and have used up the whole of the materials to form crystallised -minerals, while the latter has cooled down and solidified rapidly at -the surface, in such a way that only incipient crystallisation has -occurred, and the glassy mass has been reduced to a frothy condition -by the escape of steam-bubbles from its midst This conclusion receives -the strongest support from the fact that examples of every stage of -the change, between the glassy condition of pumice and the crystalline -condition of granite, may be detected among the materials of which the -globe is built up. - -There is still another class of facts which may be adduced in -support of the same conclusion. Many lavas, as we have seen, contain -crystals of much larger dimensions than those constituting the mass -of the rock, which is then said to be 'porphyritic' in structure. -The porphyritically embedded crystals, when carefully examined, are -often seen to be broken and injured, and to exhibit rounded edges, -with other indications of having undergone transport. When examined -microscopically, too, they often present the cavities containing -liquids which distinguish the crystals of plutonic rocks. All the -facts connected with these porphyritic lavas point to the conclusion -that while the crystals in their groundmass have separated from the -liquefied materials near the surface, the large embedded crystal, have -been floated up from great depths within the earth's crust, where they -had been originally formed. - -[Sidenote: GRANITIC REPRESENTATIVES OF OTHER LAVAS.] - -The careful consideration of all the facts of the case leads to the -conclusion that where pumice, obsidian, and rhyolite are now being -ejected at the surface, the materials which form these substances -are, at various depths in the earth's interior, slowly consolidating -in the form of quartz-felsite, granite-porphyry and granite. It may -be that we can nowhere point to the example of a mass of rock which -can be traced from subterranean regions to the surface, and is, under -such conditions, actually seen to pass from the dense and crystalline -condition of granite to the vesicular and glassy form of pumice; -but great granitic masses often exhibit a more coarsely crystalline -condition in their interior, and the offshoots and dykes which they -give off not infrequently assume the form of quartz-felsite; while, -on the other hand, the more slowly consolidated rocks found in the -interior of some rhyolite masses are not distinguishable in any way -from some of the true quartz-felsites. - -That which is true of the lavas of acid composition is equally true -of the lavas of intermediate and basic character. The andesites, -the trachytes, the phonolites, and the basalts have all their exact -representatives among the plutonic rocks, and these have a perfectly -crystalline or granitic structure. The plutonic and the volcanic -representatives of each of these groups are identical in their chemical -composition, and numerous intermediate gradations can be found between -the most completely granitic and the most perfectly vitreous or glassy -types. In illustration of this fact, we may again refer to the series -of microscopic sections of rocks given in the frontispiece. - -Another objection to the conclusion that the volcanic products of -earlier periods of the earth's history were identical in character with -those which are being ejected at the present day is based on the fact -of the supposed non-existence of the scoriaceous and glassy materials -which abound in the neighbourhood of the active volcanic vents. Where, -it is asked, do we find among the older rocks of the globe the heaps of -lapilli, dust, and scoriæ, with the glassy and pumiceous rocks that now -occur so abundantly in all volcanic districts? - -In reply to this objection, we may point out that these accumulations -of loose materials are of such a nature as to be capable of easy -removal by denuding agents, and that as they are formed upon the land -they will, if not already washed away by the action of rain, floods, -rivers, &c., run great risk of having their materials distributed, -when the land sinks beneath the waters of the ocean and the surface -is covered by new deposits. With respect to the glassy rocks it must -be remembered that the action of water, containing carbonic acid and -other substances, in percolating through such masses has a tendency -to set up crystalline action, and these glassy rocks easily undergo -'devitrification'; it would therefore be illogical for us to expect -glassy rock-masses to retain their vitreous character through long -geological periods, during which they have been subjected to the action -of water and acid gases. - -But careful observation has shown that the scoriaceous and vitreous -rocks are by no means absent among the igneous materials ejected during -earlier periods of the earth's history. Their comparative infrequency -is easily accounted for when we remember, in the first place, the ease -with which such materials would be removed by denuding forces, and in -the second place, the tendency of the action of percolating water to -destroy their characteristic features, by filling up their vesicles -with crystalline products and by effecting devitrification in their -mass. - -[Sidenote: SIMILARITY OF ANCIENT AND RECENT LAVAS.] - -If we go back to the very oldest known rock-masses of the globe, those -which are found underlying the fossiliferous Cambrian strata, we find -abundant evidence that volcanic action took place during the period -in which these materials were being accumulated. Thus, in the Wrekin, -as Mr. Allport has so well shown, we find clear proofs that before -the long-distant period of the Cambrian, there existed volcanoes -which ejected scoriæ, lapilli, and volcanic dust, and also gave rise -to streams of lava exhibiting the characteristic structures found in -glassy rocks. In these rocks, which have undergone a curious alteration -or devitrification, we still find all those peculiar structures--the -sphærulitic, the perlitic, and the banded--so common in the rhyolites -of Hungary, with which rocks the Wrekin lavas, in their chemical -composition, precisely agree. Prof. Bonney, too, has shown that the -rocks of Charnwood Forest, which are also probably of pre-Cambrian age, -contain great quantities of altered volcanic agglomerates, tuffs, and -ashes. I have found the sphærulitic, perlitic, and banded structures -exhibited by British lavas of the Cambrian, Silurian, Devonian and -Carboniferous periods, as well as in those of Tertiary age; and in -connection with these different lavas we find vast accumulations, -sometimes thousands of feet in thickness, of volcanic agglomerates and -tuffs which have undergone great alteration. - -All these facts point to one conclusion--namely, that during all past -geological periods, materials similar to those which are now being -extruded from volcanic vents were poured out on the earth's surface -by analogous agencies. If we could trace the lava-streams of the -present day down to the great subterranean reservoirs from which their -materials have been derived, we should doubtless find that at gradually -increasing depths, where the pressure would be greater and the escape -of heat from the mass slower, the rocky materials would by degrees -assume more and more crystalline characters. We should thus find -obsidian or rhyolite insensibly passing into quartz-felsite and finally -into granite; trachyte passing into orthoclase-porphyry and syenite; -and basalt passing into dolerite, augite-porphyry, and gabbro. - -On the other hand, if we could replace the great masses of stratified -rocks which must once have overlain the granites, syenites, diorites, -and gabbros, we should find that, as we approached the original -surface, these igneous materials would gradually lose their crystalline -characters, and when they were poured out at the surface would take the -forms of rhyolite, trachyte, andesite, and basalt--all of which might -occasionally assume the glassy forms known as obsidian or tachylyte. - -[Sidenote: ALTERED FORMS OF ANCIENT LAVAS.] - -But while we insist on the essential points of similarity between -the lavas poured out upon the surface of the earth during earlier -geological periods and those which are being extruded at the present -day, we must not forget that by the action of percolating water and -acid gases, the mineral constitution, the structure, and sometimes -even the chemical composition of these ancient lavas may undergo -a vast amount of change. In not a few cases these changes in the -characters of a lava may be carried so far that the altered rock bears -but little resemblance to the lava from which it was formed, and it may -be found desirable to give it a new name. Among the rocks of aqueous -origin we find similar differences in the materials deposited at -different geological periods. Clay, shale and clay-slate have the same -composition, and the two latter are evidently only altered forms of the -first mentioned, yet so great is the difference in their characters -that it is not only allowable, but desirable, to give them distinctive -names. - -In the same way, among the deposits of the earlier geological periods -we find rocks which were doubtless originally basalts, but in which -great alterations have been produced by the percolation of water -through the mass. The original rock has consisted of crystals of -felspar, augite, olivine, and magnetite distributed through a glassy -base. But the chemical action of water and carbonic acid may have -affected all the ingredients of the rock. The outward form of the -felspar crystals may be retained while their substance is changed to -kaolinite, various zeolites, and other minerals; the olivine maybe -altered to serpentine and other analogous minerals; the magnetite -changed to hydrous peroxide of iron; the augite may be changed to -uralite or hornblende; and the surrounding glassy mass more or -less devitrified and decomposed. The hard, dense, and black rock -known as basalt has under these circumstances become a much softer, -earthy-looking mass of a reddish-brown tint, and its difference from -basalt is so marked that geologists have agreed to call it by another -name, that of 'melaphyre.' Even in their ultimate chemical compositions -the 'melaphyres' differ to some extent from the basalts, for some of -the materials of the latter may have been removed in solution, and -water, oxygen, and carbonic acid have been introduced to combine with -the remaining ingredients. - -But if we carefully study, by the aid of the microscope, a large series -of basalts and melaphyres, we shall find that many rocks of the former -class show the first incipient traces of those changes which would -reduce them to the latter class. Indeed, it is quite easy to form a -perfect series from quite unaltered basalts to the most completely -changed melaphyres. Hence we are justified in concluding that all the -melaphyres were originally basalts, just as we infer that all oaks were -once acorns. - -Now changes, similar to those which we have seen to take place in -the case of basaltic lavas, are exhibited by the lavas of every -other class, which have been exposed to the influence of the same -agencies,--namely, the passage of water and acid gases. But inasmuch as -the minerals composing the basic lavas are for the most part much more -easily affected by such agencies than are the minerals of acid lavas, -the ancient basic rocks are usually found in a much more highly altered -condition than are the acid rocks of equivalent age. - -[Sidenote: NAMES GIVEN TO ALTERED LAVAS.] - -We thus see that each of the classes of modern lavas has its -representative in earlier geological periods, in the form of rocks -which have evidently been derived from these lavas, through alterations -effected by the agency of water and acid-gases that have permeated -their mass. Thus, while the basalts are represented among the ancient -geological formation by the melaphyres, the andesites are represented -by the porphyrites, and the trachytes and rhyolites by different -varieties of felstones. And, as we can form perfect series illustrating -the gradual change from basalt to melaphyre, so we can arrange other -series demonstrating the passage of andesites into porphyrites, and of -trachytes and rhyolites into felsites. - -It must be remembered, however, that these changes do not take place -in anything like determinate periods of time. Occasionally we may -find lavas of ancient date which have undergone surprisingly little -alteration, and in other cases there occur lavas belonging to a -comparatively recent period which exhibit very marked signs of change. - -The alteration of the lavas and other igneous rocks does not, however, -stop with the production of the melaphyres, porphyrites, and felstones. -By the further action of the water and carbonic acid of the atmosphere, -the basic lavas are reduced to the soft earthy mass known as 'wacke,' -and the intermediate and acid lavas to the similar material known as -'claystone.' As the passage of water and carbonic acid gas through -these rock-masses goes on, they are eventually resolved into two -portions, one of which is insoluble in water and the other is soluble. -The insoluble portion consists principally of quartz, the crystals -of which are almost unattacked by water and carbonic acid, and the -hydrated silicate of alumina. All the sands and clays, which together -make up more than nine-tenths of the stratified rocks of the globe, -are doubtless derived, either directly or indirectly, from these -insoluble materials separated during the decomposition of volcanic and -plutonic rocks. The soluble materials, which consist of the carbonates, -sulphates and chlorides of lime, magnesia, soda, potash, and iron, -give rise to the formation of the limestones, gypsum, rock-salt, -ironstones, and other stratified masses of the earth's crust. We thus -see how the igneous materials of the globe, by their decomposition, -famish the materials for the stratified rock-masses. The relations of -the different plutonic and volcanic rocks to one another and to the -materials which are derived from them are illustrated in the following -table. - -[Sidenote: RELATIONS OF ALTERED TO UNALTERED LAVAS.] - - Unaltered Altered Decomposed - Plutonic Rocks lavas lavas Rocks - - Granite {Quartz-felsite } Rhyolite and } - { ('quartz-porphyry')} Obsidian } Felstone } - } } - Syenite {Orthoclase-porphyry } Trachyte } } Claystones - } - Diorite {Hornblende-porphyry } Andesite Porphyrite } - - Miascite {Liebnerite porphyry } Phonolite ? -- - - Gabbro {Augite-porphyry and } Basalt Melaphyre Wacke - {Dolerite } - -Some petrographers, indeed, have maintained the principle that rocks -belonging to widely separated geological periods, even when they -exhibit no essential points of difference, should nevertheless be -called by distinct names. But such a system of classification is -calculated rather to hinder than to advance the cause of science. If -the palæontologist were to adopt the same principle and give distinct -names to the same fossil, when it was found to occur in two different -geological formations, we can easily understand what confusion would -be occasioned, and how the comparison of the fauna and flora of the -different formations would be thereby rendered impossible. But the -naturalist, in his diagnosis of a species, wisely confines himself -to the structure and affinities of the organism before him; and in -the same way the petrographer, in giving a name to a rock, ought -to be guided only by his studies of its chemical composition, its -mineralogical constitution, and its structure, putting altogether -out of view its geographical distribution and geological age. Only -by strict attention to this principle can we hope to arrive at such -comparisons of the rocks of different areas and different periods, as -may serve as the basis for safe inductions. - -Before leaving this question of the relation which exists between the -igneous rocks of different ages, it may be well to notice several facts -that have been relied upon, as proving that the several geological -periods are distinguished by characteristic igneous products. - -It has frequently been asserted that the acid igneous rocks are present -in much greater quantities in connection with the older geological -formations than axe the basic; while, on the other hand, the basic -igneous rocks are said to have been extruded in greater abundance in -the more recent geological periods. But in considering this question it -must not be forgotten that, as a general rule, the basic rocks undergo -decomposition and disintegration far more rapidly than do the acid -rocks. In consequence of this circumstance the chance of our finding -their recognisable representatives among the older formations, is much -less in the case of the former class of rocks than in the latter. As a -matter of fact, however, we do find great masses of gabbro, diabase, -and melaphyre associated even with the oldest geological formations, -while trachytes and rhyolites abound in many volcanic districts where -active vents exist at the present day. Upon a general review of the -subject, it may well be doubted whether the supposed preponderance of -acid igneous materials in the earlier periods of the earth's history, -and of basic igneous materials during the later periods, rests on any -substantial basis of observation. - -[Sidenote: AUGITIC AND HORNBLENDIC ROCKS.] - -Another difference which has frequently been relied upon, as -distinguishing the older igneous rocks from those of more recent date, -is the supposed fact that the former are characterised by the presence -of hornblende, the latter by the presence of augite. It may be admitted -that this distinction is a real one, but its significance and value are -greatly diminished when we remember the relations which exist between -the two minerals in question. Hornblende and augite are interesting -examples of a dimorphous substance; in chemical composition they are -identical, or rather they are liable to variation between the same -limits, but in their crystalline forms and optical characters they -differ from one another. It has been proved that hornblende is the -stable, and augite the unstable condition of the substance in question. -If hornblende be fused and allowed to cool, it crystallises in the form -of augite. On the other hand, augite-crystals in rocks of ancient date -are found undergoing gradual change and passing into hornblende. The -mineral uralite has the outward form of augite, but the cleavage and -optical properties of hornblende; and there are not wanting many facts -pointing to the conclusion that rocks which now contain hornblende were -originally augitic masses, in which the unstable mineral in their -midst has been gradually converted into the stable one. - -There are, however, two minerals which up to the present time have -been found in association only with the older and newer rock-masses -respectively. These are _muscovite_, or the white form of mica, which -occurs in so many granites, but has not yet been discovered in any -modern representative of that rock; and leucite, which is not yet known -in rocks of older date than the Tertiary. - -When we remember that muscovite would appear to be a product of -deep-seated igneous action, and is only found in rock-masses that have -been formed under such conditions, we shall be the less surprised at -its non-occurrence in rocks of recent date, especially if we bear in -mind the fact that very few of the younger granitic rocks have as yet -been exposed at the surface by denudation. - -With respect to leucite, on the other hand, it must be remembered that -it is a very unstable mineral which appears to be easily changed into -felspar. It is by no means improbable, therefore, that some ancient -igneous rocks which now contain felspar were originally leucitic rocks. - -To the view that the action of volcanic forces upon the globe during -past geological times was similar in kind to that which we now observe -going on around us, still another objection has been raised. It has -been asserted that some of the deposits of igneous rock associated -with the older geological formations are of such a nature that they -could not possibly have been accumulated around volcanic vents of the -kind which we see in operation around us. - -[Sidenote: VOLCANIC ORIGIN OF ANCIENT IGNEOUS ROCKS.] - -Mr. Mallet has declared that the igneous products of the Palæozoic -period differ fundamentally in character from those materials formed -by volcanic action during the later Secondary and the Tertiary -periods. Upon what observations these generalisations are based he -has given us no information, and the enormous mass of facts which -have been collected in recent years concerning the structure of the -lavas and fragmental volcanic deposits of the pre-Cambrian, Cambrian, -Silurian, Devonian and Carboniferous periods, all point to a directly -opposite conclusion. The more carefully we carry on our investigations -concerning these ancient lavas, by the aid of chemical analysis and -microscopic study, the more are we convinced of the essential identity -of the ancient and modern volcanic rocks, both in their composition -and their minute structure. Of great masses of dust produced by -crushing, such as Mr. Mallet has supposed to have been formed during -the earlier geological periods, there is not the smallest evidence; -but we everywhere find proofs, when the rocks are minutely examined, -of the vesicular structure so characteristic of materials produced by -explosive volcanic action. - -It has frequently been asserted that in the great districts covered by -basaltic lavas which we find in the Rocky Mountains of North America, -in the Deccan of India, in Abyssinia, and even in the Western Isles of -Scotland, we have proofs of the occurrence, during earlier geological -periods, of volcanic action very different in character from that -which at present takes place on our globe. It has been asserted that -the phenomena observed in these districts can only be accounted for by -supposing that great fissures have opened in their midst, from which -lavas have issued in enormous floods unaccompanied by the ordinary -explosive phenomena of volcanoes. - -It must be remembered, however, that none of the districts in question -have been subjected to careful and systematic examination with a view -to the discovery of the vents from which these masses of lava have -issued, with the exception of that which occurs in our own islands. In -this case, in which superficial observers have spoken of the district -as being covered with horizontal lava-sheets piled upon one another to -the depth of 3,000 feet, careful study of the rock-masses has shown -that the accumulations of basalt really consist of a great number of -lava-currents which have issued at successive epochs covering enormous -periods of time. During the intervals between the emission of these -successive lava-currents the surfaces of the older ones have been -decomposed, and formed soils upon which forests have grown up; they -have been eroded by streams, the valleys so formed being filled with -gravels; and lakes have been originated on their surfaces in which -various accumulations have taken place. - -[Sidenote: TRULY-VOLCANIC ORIGIN OF LAVA PLATEAUX.] - -It has been demonstrated, moreover, that the basal-wrecks of no less -than five volcanic mountains, each of which must have rivalled Etna in -its proportions, existed within this area, and the connection of the -lava-currents, which have deluged the surrounding tracts, with these -great volcanoes has been clearly proved. It is probable that when -more careful and systematic researches are carried on in the other -districts, in which widely-spread sheets of basaltic rocks exist, -similar volcanic vents will be discovered. It must also be remembered -that if such a country as Iceland were subjected to long-continued -denudation, the mountain peaks and cones of loose materials would be -worn away, the whole island being thus reduced to a series of plateaux -composed of lava-sheets, the connection of which with the crystalline -materials filling the great volcanic vents, a superficial observer -might altogether fail to recognise. - -But even where we cannot trace the former existence of great volcanic -mountains, like those which once rose in the Hebrides, it would -nevertheless be very rash to conclude that the vast plateaux of -lava-rock must have been formed as gigantic floods unaccompanied by -ordinary volcanic action. Mr. Darwin has pointed out that in crossing -districts covered by lava, he was frequently only able to determine -the limits of the different currents of which it was made up, by an -examination of the age of the trees and the nature of the vegetation -which had sprung up on them. And everyone who has travelled much in -volcanic districts can confirm this observation; what appears at first -sight to be a great continuous sheet of lava proves upon more careful -observation to be composed of a great number of distinctly different -lava-currents, which have succeeded one another at longer or shorter -intervals. - -We must remember, too, how various in kind are the volcanic -manifestations which present themselves under different circumstances. -Sometimes the amount of explosive action at a volcanic vent is very -great, and only fragmental ejections take place, composed of the frothy -scum of the lava produced by the escape of gases and vapours from its -midst. But in other cases the amount of explosive action may be small, -and great volumes of igneous materials may issue as lava-streams. In -such cases, only small scoriæ-cones would be formed around the vents, -and one half of such cones is commonly swept away by the efflux of the -lava-currents, while the remainder may be easily removed by denuding -action or be buried under the lava-currents issuing from other vents -in the neighbourhood. Thus it may easily come to pass that what a -superficial observer takes for an enormous mass of basaltic lava poured -out from a great fissure at a single effort, may prove upon careful -observation to be made up of innumerable lava-currents, each of which -is of moderate dimensions; and it may further be found that these -lava-currents, instead of being the product of a single paroxysmal -effort from one great fissure, have been accumulated by numerous small -outbursts taking place at wide intervals, from a great number of minor -orifices. - -[Sidenote: SHIFTING OF VOLCANIC ACTION IN DIFFERENT AREAS.] - -Having then considered the arguments which have been adduced in support -of the view that the volcanic phenomena of former geological periods -differ from those which are still occurring upon the globe, we may -proceed to state the general conclusions which have been drawn from the -study of the volcanic rocks of the different geological periods. - -From a survey of the volcanic rocks of different ages, we are led to -the interesting and important conclusion that the scene of volcanic -action has been continually shifting to fresh areas at different -periods of the earth's history. We find repeated proofs that the -volcanic energy has made its appearance at a certain part of the -earth's crust, has gradually increased in intensity to a maximum, and -then as slowly declined. But as these manifestations have died away -at one part of the earth's surface, they have gradually made their -appearance at another. In every district which has been examined, we -find abundant proofs that volcanic energy has been developed at certain -periods, has disappeared during longer or shorter periods, and then -reappeared in the same area. And on the other hand, we find that there -is no past geological period in which we have not abundant evidence -that volcanic outbursts took place at some portion of the earth's -surface. - -To take the case of our own islands for example. We know that during -the pre-Cambrian periods volcanic outbursts occurred, traces of which -are found both in North and South Wales, in the Wrekin Chain in -Shropshire, in Charnwood Forest, and in parts of Scotland and Ireland. - -In Cambro-Silurian times we have abundant proofs, both in North Wales -and the Lake district, that volcanic action on the very grandest -scale was taking place during the Arenig and the older portion of -the Llandeilo periods, and again during the deposition of the Bala -or Caradoc beds. The lavas, tuffs, and volcanic agglomerates ejected -during these two periods have built up masses of rock many thousands of -feet in thickness. Snowdon and Cader Idris among the Welsh mountains, -and some of the higher summits of the Lake district, have been carved -by denudation from the vast piles of volcanic materials ejected during -these periods. - -In Devonian or Old-Red-Sandstone times, volcanic activity was renewed -with fresh violence upon that part of the earth's surface now occupied -by the British Islands. Along the line which now forms the Grampians -there rose a series of volcanoes of the very grandest dimensions. Ben -Nevis, and many others among the higher Scotch mountains, have been -carved by denudation from the hard masses of granite, quartz-felsite, -and other plutonic rocks which formed the central cores of these -ancient volcanic piles. The remains of the great lava-sheets, and of -the masses of volcanic agglomerate ejected from these grand Devonian -volcanoes, make up hill-ranges of no mean altitude, like the Sidlaws, -the Ochils, and the Pentlands. - -[Sidenote: ANCIENT BRITISH VOLCANOES.] - -The volcanic action of the Devonian period was prolonged into -Carboniferous times, but was then evidently diminishing gradually in -violence. Instead of great central volcanoes, such as existed in the -earlier period, we find innumerable small vents which threw out tuffs, -agglomerates and lavas, and were scattered over the districts lying -around the bases of the now extinct Devonian volcanoes. In the central -valley of Scotland and in many parts of England, we find abundant -proofs of the existence of these small and scattered volcanic vents -during Carboniferous times. The well-known hill of Arthur's Seat, which -overlooks the city of Edinburgh, and many castle-crowned crags of the -Forth and Clyde valleys, are the worn and denuded relics of these small -volcanoes. There are some indications which point to the conclusion -that the volcanic action of the Newer Palæozoic epoch had not entirely -died out in Permian times, but the evidence upon this point is not -altogether clear and satisfactory. - -During nearly the whole of the Secondary or Mesozoic periods the -volcanic forces remained dormant in the area of the British Isles. -Some small volcanic outbursts, however, appear to have occurred in -Triassic times in Devonshire. But in other areas, such as the Tyrol, -South-eastern Europe and Western America, the Triassic, Jurassic, and -Cretaceous periods were marked by grand manifestations of volcanic -activity. - -The volcanic forces which had during the long Mesozoic periods deserted -our part of the earth's surface, appear to have returned to it in -full rigour in the Tertiary epoch. In the Newer-Palæozoic periods -the direction of the great volcanic band which traversed our islands -appears to have been from north-east to south-west; but in Tertiary -times a new set of fissures were opened running from north to south. -There is evidence that during the Eocene or Nummulitic period, the -first indications of the subterranean forces having gathered strength -below the district were afforded by the issue of calcareous and -siliceous springs, and soon fissures were opened which emitted scoriæ, -tuffs, and lavas. The intensity of the volcanic action gradually -increased till it attained its maximum in the Miocene period, when -a great chain of volcanic mountains stretched north and south along -the line of the Inner Hebrides, the north-east of Ireland, and the -sea which separates Great Britain from Ireland. The basal-wrecks of a -number of these volcanoes can be traced in the islands of Skye, Mull, -Rum, and parts of the adjoining mainland. We have already seen that -along this great band of volcanic action, which traverses the Atlantic -Ocean from north to south, a number of active vents still exist, though -their energy is now far less intense than was the case in former times. -The only vestiges of the action of these now declining volcanic forces, -at present found in our islands, are the hot springs of Bath and a few -other warm and mineral springs; but in connection with this subject it -must be remembered that our country occasionally participates in great -earthquake-vibrations, like that which destroyed Lisbon in the year -1759. - -[Sidenote: ANCIENT VOLCANOES IN OTHER DISTRICTS.] - -If we were to study any other part of the earth's surface, we should -arrive at precisely the same conclusion as those to which we have -been conducted by our examination of the British Islands--namely, -that during past geological times the subterranean forces had made -themselves felt in the area, had gradually attained a maximum, and then -as gradually declined, passing through all those varied cycles which we -have described in a former chapter. And we should also find that these -periods of volcanic activity alternated with other periods of complete -quiescence which were of longer or shorter duration. But on comparing -two different districts, we should discover that what was a period of -volcanic activity in the one was a period of repose in the other, and -_vice versâ_. - -From these facts geologists have been led to the conclusion which we -have already enunciated--namely, that the subterranean forces are in a -state of continual flux over the surface of the globe. At one point of -the earth's crust these forces gradually gather such energy as to rend -asunder the superincumbent rock-masses and make themselves manifest -at the surface in the series of phenomena characteristic of volcanic -action. But after a longer or shorter interval of time--an interval -which must probably be measured by millions of years--the volcanic -forces die out in that area to make their appearance in another. - -Hence, although we may not be able to prove the fact by any -mathematical demonstration, a strong presumption is raised in favour -of the view that the subterranean energy in the earth's crust is a -constant quantity, and that the only variations which take place are in -the locality of its manifestation. - -Upon this question whether the amount of this subterranean energy -within the earth's crust is at the present time increasing, stationary, -or declining, we are not altogether destitute of evidence. There are -some considerations connected with certain astronomical hypotheses, to -which we shall hereafter have to refer, that might lead us to entertain -the view that the subterranean activity was once far greater than it -is at present, and that during the long periods of the earth's past -history it has been slowly and gradually declining. And those who -examine the vast masses of igneous materials which have been poured -out from volcanic vents during the earlier periods of the earth's -history may be inclined, at first sight, to point to them as affording -conclusive proof of this gradual decline. - -[Sidenote: SUPPOSED DECLINE OF VOLCANIC ACTION.] - -But a more careful study of the rocks in question will probably cause -a geologist to pause before jumping to such a conclusion. If we look -at the vast masses of volcanic materials erupted in Miocene times in -our own island and in Ireland, for example, we might be led to imagine -that we have the indications of a veritable 'Reign of Fire,' and that -the evidence points to a condition of things very different indeed -from that which prevails at the present day. But in arriving at such -a conclusion we should be neglecting a most important consideration, -the disregard of which has been the fertile parent of many geological -errors. Many independent lines of evidence all point to the inference -that these volcanic ejections are not the result of one violent effort, -but are the product of numerous small outbreaks which have been -scattered over enormous periods of time. - -When we examine with due care the lavas, tuffs, and other volcanic -ejections which constitute such mountain-masses as those of the -Hebrides, of the Auvergne, and of Hungary, we find clear proofs -that the ancient Miocene volcanoes of these districts were clothed -with luxuriant forests, through which wild animals roamed in the -greatest abundance. The intervals between the ejections of successive -lava-streams were often so great, that soils were formed on the -mountain-slope, and streams cut deep ravines and valleys in them. - -The island of Java is situated near the very heart of what is at the -present day the most active volcanic centre on the face of the globe, -yet vegetable and animal life flourish luxuriantly there, and the -island is one of the richest and most fertile spots upon the face of -the globe. Not all the terrors of occasional volcanic outbreaks will -ever drive the Neapolitan vine-dressers from the fertile slopes of -Vesuvius, for its periods of repose are long, and its eruptions are of -short duration. - -These considerations lead the geologist to conclude that the evidence -afforded by the ancient volcanic rocks is clear and positive in support -of the view that the manifestations of the subterranean forces in the -past agree precisely in their nature and in their products with those -taking place around us at the present time. On the question of great -secular changes having occurred in the amount of volcanic energy in -past geological periods, the evidence must be pronounced negative, or -at the best doubtful. - -But even if the geologist confesses himself unable to establish the -fact of any decline in the subterranean energies during the vast -periods of which he takes cognisance, it must be remembered that such -decline may really be going on; for vast as was the duration of the -geological epochs, they probably constitute but a fraction of those far -grander periods which are required by the speculations of the physical -astronomer. - - - - -CHAPTER X. - -THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE. - - -The first impression which is produced upon the mind, when the -phenomena of volcanic action are studied, is that here we have -exhibitions of destructive violence the effects of which must be -entirely mischievous and disastrous to the living beings occupying the -earth's surface. A little consideration will convince us, however, that -the grand and terrible character of the displays of volcanic energy -have given rise to exaggerated notions concerning their destructive -effects. The fact that districts situated over the most powerful -volcanic foci, like Java and Japan, are luxuriant in their productions, -and thickly inhabited, may well lead us to pause ere we condemn -volcanic action as productive only of mischief to the living beings -on the earth's surface. The actual slopes of Vesuvius and Etna, and -many other active volcanoes, are abundantly clothed with vineyards and -forests and are thickly studded with populous villages. - -As a matter of fact, the actual amount of damage to life and property -which is effected by volcanic eruptions is small. Usually, the -inhabitants of the district have sufficient warning to enable them -to escape with their lives and to carry away their most valuable -possessions. And though fertile tracts are covered by loose dust and -ashes, or by lava- and mud-currents, yet the sterility thus produced -is generally of short duration, for by their decomposition volcanic -materials give rise to the formation of the richest and most productive -soils. - -Earthquakes, as we have already seen, are far more destructive in -their effects than are volcanoes. Houses and villages, nay even entire -cities, are, by vibrations of portions of the earth's crust, reduced to -heaps of ruins, and famines and pestilences too frequently follow, as -the consequence of the disorganisation of our social systems by these -terrible catastrophes. - -It may well be doubted, however, whether the annual average of -destruction to life and property caused by all kinds of subterranean -action, exceeds that produced either by floods or by hurricanes. Yet -we know that the circulation of water and air over our globe are -beneficial and necessary operations, and that the mischief occasionally -wrought by the moving bodies of water and air is quite insignificant -compared with the good which they effect. - -In the same way, we shall be able to show that the subterranean -energies are necessary to the continued existence of our globe as -a place fitted for the habitation of living beings, and that the -mischievous and destructive effects of these energies bear but a small -and insignificant proportion to the beneficial results with which they -must be credited. - -[Sidenote: LEVELLING ACTION OF DENUDING FORCES.] - -We have had frequent occasion in the preceding pages to refer to the -work--slow but sure, silent but effective--wrought by the action of -the denuding forces ever operating upon the surface of our globe. The -waters condensing from the atmosphere and falling upon the land in -the form of rain, snow, or hail, are charged with small quantities of -dissolved gases, and these waters penetrating among the rock-masses -of which the earth's crust is composed, give rise to various -chemical actions of which we have already noticed such remarkable -illustrations in studying the ancient volcanic products of our globe. -By this action the hardest and most solid rock-masses are reduced -to a state of complete disintegration, certain of their ingredients -undergoing decomposition, and the cementing materials which hold -their particles together being removed in a state of solution. In the -higher regions of the atmosphere this work of rock-disintegration -proceeds with the greatest rapidity; for there the chemical action is -reinforced by the powerful mechanical action of freezing water. On -high mountain-peaks the work of breaking up rock-masses goes on at the -most rapid rate, and every craggy pinnacle is swathed by the heaps -of fragments which have fallen from it. The Alpine traveller justly -dreads the continual fusillade of falling rock-fragments which is -kept up by the ever-active power of the frost in these higher regions -of the atmosphere; and fears lest the vibrations of his footsteps -should loosen, from their position of precarious rest, the rapidly -accumulating piles of detritus. No mountain-peak attains to any very -great elevation above the earth's surface, for the higher we rise in -the atmosphere the greater is the range of temperature and the more -destructive are the effects of the atmospheric water. The moon, which -is a much smaller planet than our earth, has mountains of far greater -elevation; but the moon possesses neither an atmosphere nor moisture on -its surface, to produce those levelling effects which we see everywhere -going on around us upon the earth. - -The disintegrated materials, produced by chemical and mechanical -actions of the atmospheric waters upon rock-masses, are by floods, -rivers, and glaciers, gradually transported from higher to lower -levels; and sooner or later every fragment, when it has once been -separated from a mountain-top, must reach the ocean, where these -materials are accumulated and arranged to form new rocks. - -Over every part of the earth's surface these three grand operations of -the disintegration of old rock-masses, the transport of the materials -so produced to lower levels, and the accumulation of these materials -to form new rocks, is continually going on. It is by the varied action -of these denuding agents upon rocks of unequal hardness, occupying -different positions in relation to one another, that all the external -features of hills, and plains, and mountains owe their origin. - -It is a fact, which is capable of mathematical demonstration, that by -the action of these denuding forces the surface of all the lands of the -globe is being gradually but surely lowered; and this takes place at -such a rate that in a few millions of years the whole of the existing -continents must be washed away and their materials distributed over the -beds of the oceans. - -[Sidenote: NECESSITY FOR COMPENSATING AGENCIES.] - -It is evident that there exists some agency by which this levelling -action of the denuding forces of the globe is compensated; and a little -consideration will show that such compensating agency is found in the -subterranean forces ever at work within the earth's crust. The effects -of these subterranean forces which most powerfully arrest our attention -are volcanic outbursts and earthquake shocks, but a careful study of -the subject proves that these are by no means the most important of the -results of the action of such forces. Exact observation has proved that -almost every part of the earth's surface is either rising or falling, -and the striking and destructive phenomena of volcanoes and earthquakes -probably bear only the same relation to those grand and useful -actions of the subterranean forces, which floods do to the system of -circulating waters, and hurricanes to the system of moving air-currents. - -If we ride in a well-appointed carriage with good springs, upon -a railway which is in excellent order, the movement is almost -imperceptible to us; and the rate of speed may be increased -indefinitely, without making itself apparent to our senses. The -smallest impediment to the evenness of the movement--such as that -produced by a small object placed upon the rails--at once makes itself -felt by a violent jar and vibration. How perfectly insensible we may -be of the grandest and most rapid movements is taught us by the facts -demonstrated by the astronomer. By the earth's daily rotation, we are -borne along at a rate which in some places amounts to over 1,000 miles -an hour; and by its annual revolution we are every hour transported -through a distance of 70,000 miles; yet concerning the fact and -direction of these movements we are wholly unconscious. - -In the case both of the railway train and of our planet, we can only -establish the reality of the movement, and its direction and rate, by -means of observations upon external objects, which appear to us to -have a movement in the opposite direction. In the same way we can only -establish the fact of the movement of portions of the earth's crust -by noticing the changing positions of parts of the earth's surface in -relation to the constant level of the ocean. When this is done we find -abundant proof that while some parts of the earth's crust are rising, -others are as undoubtedly undergoing depression. - -[Sidenote: POTENCY OF THE SUBTERRANEAN FORCES.] - -We shall be able to form some idea of the vastness of the effects -produced by the subterranean forces, by a very simple consideration. -It is certain that during the enormous periods of time of which the -records have been discovered by the geologist, there have always been -continents and oceans upon the earth's surface, just as at present, -and it is almost equally certain that the proportions of the earth's -surface occupied by land and water respectively, have not varied very -widely from those which now prevail. But, at the same time, it is an -equally well-established bet that the denuding forces ever at work -upon the earth's surface would have been competent to the removal -of existing continents many times over, in the vast periods covered -by geological records. Hence we are driven to conclude that the -subterranean movements have in past times entirely compensated for the -waste produced by the denuding forces ever at work upon our globe. But -this is not all. The subterranean forces not only produce upheaval; -in a great many cases the evidences of subsidence are as clear and -conclusive as are those of upheaval in others. Hence we are driven to -conclude that the forces producing upheaval of portions of the earth's -crust are sufficient, not only to balance those producing subsidence, -but also to compensate for the destructive action of denuding agents -upon the land-masses of the globe. - -It is only by a careful and attentive study and calculation of the -effects produced by the denuding agents at work all around us, aided -by an examination of the enormous thicknesses of strata formed by the -action of such causes during past geological times, that we are able -to form any idea of the reality and vastness of the agents of change -which are ever operating to modify the earth's external features. When -we have clearly realised the grand effects produced on the surface -of the globe by these external forces, through the action of its -investing atmosphere and circulating waters, then, and only then, -shall we be in a position to estimate the far greater effects resulting -from the internal forces, of which the most striking, but not the most -important, results are seen in the production of volcanic eruptions and -earthquake-shocks. - -Another series of facts which serve to convince the geologist of the -reality and potency of the forces ever at work within the earth's -crust, and the way in which these have operated during past geological -periods, is found in the disturbed condition of many of the stratified -rock-masses of which it is composed. Such stratified rock-masses, it is -clear, must have been originally deposited in a position of approximate -horizontality; but they are now often found in inclined and even -vertical positions; they are seen to be bent, crumpled, puckered, and -folded in the most remarkable manner, and have not unfrequently been -broken across by dislocations--'faults'--which have sometimes displaced -masses, originally in contact, to the extent of thousands of feet. -The slate-rocks of the globe, moreover, bear witness to the fact that -strata have been subjected to the action of lateral compression of -enormous violence and vast duration; while in the metamorphic rocks -we see the effects of still more extreme mechanical strains, which -have been in part transformed into chemical action. No one who has -not studied the crushed, crumpled, fractured, and altered condition -of many of the sedimentary rocks of the globe, can form the faintest -idea of the enormous effects of the internal forces which have been in -operation within the earth's crust during earlier geological periods. -And it is only by such studies as these that we at last learn to regard -the earthquake and volcanic phenomena of our globe, not as the grandest -and most important effects of these forces, but as their secondary -and accidental accompaniments. 'Volcanoes,' it has been said, 'are -the safety-valves of the globe;' and when we come to realise the real -extent and nature of the internal forces ceaselessly working in the -earth's crust we shall scarcely be disposed to regard the simile as an -overstrained one. - -[Sidenote: RELATION TO CONTINENTAL MOVEMENTS.] - -The first geologist who attempted to show the exact relations existing -between those subterranean forces which cause the movements of -continental masses of land, and those more startling displays of energy -which are witnessed in volcanic outbursts, was the late Mr. Poulett -Scrope. At a somewhat later date Mr. Darwin, in his remarkable paper -'On the Connexion of certain Volcanic Phenomena in South America, and -on the Formation of Mountain-chains and Volcanoes as the effect of -Continental Elevations,' threw much new and important light upon the -question. - -While, on the one hand, we are led by recent geological investigations -to reject the notions which were formerly accepted, by which -mountain-ranges were supposed to be suddenly and violently upheaved by -volcanic forces, we are, on the other hand, driven to conclude that -without the action of these subterranean forces, the irregularities -which are exhibited on the earth's surface could not have had any -existence. - -It is true that the actual forms of the mountain-ranges are due -directly to the action of denuding forces, which have sculptured -out from the rude rocky masses all the varied outlines of peaks and -crags, of ravines and valleys. But it is none the less true that -the determining causes which have directed and controlled all this -earth-sculpture, are found in the relative positions of hard and soft -masses of rock; but these rock-masses have acquired their hardness and -consistency, and have assumed their present positions, in obedience -to the action of subterranean forces. Hence we see that though the -formation of mountain-ranges is proximately due to the denuding -forces, which have sculptured the earth's surface, the primary cause -for the existence of such mountain-chains must be sought for in the -fact that subterranean forces have been at work, folding, crumpling, -and hardening the soft sediments, and placing them in such positions -that, by the action of denudation, the more indurated portions are left -standing as mountain-masses above the general surface. - -The old notion that mountain-chains are due to a vertical upthrust -from below, finds but little support when we come to study with due -care the positions of the rock-masses composing the earth's crust. On -the contrary, we find that mountain-ranges are usually carved out of -the crushed and crumpled edges of strata which have along certain lines -been influenced by great mechanical strains, and subjected to more or -less induration and chemical alteration. When we compare these folded -and contorted portions of the strata with those parts of the same beds -which are not so affected, we find the effects produced in the former -are not such as would result from an upthrust from below, but from -movements by which a tangential strain would be brought about. If we -imagine certain lines of weakness to exist in the solid crust of the -earth, then any movements in the portions of the crust between these -lines of weakness would cause crushing and crumpling of the strata -along the latter. - -[Sidenote: FORMATION OF MOUNTAIN-CHAINS.] - -Recent investigations of Dana and other authors have thrown much new -light upon the question of the mode of formation of mountain-chains, -and the relation between the movements by which they are produced and -the sudden and violent manifestations of force witnessed in volcanic -outbursts. We cannot, perhaps, better illustrate this subject than by -giving a sketch of the series of operations to which the great Alpine -chains owe their origin. - -There are good grounds for believing that the great mountain-axis of -Southern Europe, with its continuation in Asia, had no existence -during the earlier geological periods. Indeed, it has been proved that -all the higher among the existing mountain-chains of the globe have -been almost entirely formed in Tertiary times. The reason of this -remarkable fact is not far to seek. So rapid is the work of denudation -in the higher regions of the atmosphere, that the elevated crags and -pinnacles are being broken up by the action of moisture and frost at -an exceedingly rapid rate. This fact is attested by the existence -of those enormous masses of angular rock-fragments which are found -lodged on every vantage-ground among the mountain-summits, as well as -by the continually descending materials which are borne by glaciers -and mountain-torrents to the valleys below. Where such a rate of -disintegration as this is maintained, no elevated mountain-crests -could exist through long geological periods. It is true we find in all -parts of the globe relics of many mountain-chains which were formed -before the Tertiary period; but these have by long-continued denudation -been worn down to 'mere stumps.' Of such worn-down and degraded -mountain-ranges we have examples in the Scandinavian chains, and some -of the low mountain-regions of Central Europe and North America. - -Let us now proceed to illustrate this subject by briefly sketching the -history of that series of operations by which the great mountain-chains -of the Alpine system have been formed. - -The first stage of that grand series of operations appears from recent -geological researches to have consisted in the opening of a number -of fissures running along a line near to that at which, in a long -subsequent period, the elevation of the mountain-masses took place. -This betrayal of the existence of a line of weakness in this part of -the earth's crust occurred in the Permian period, and from that time -onward a series of wonderful movements and changes have been going -forward, which have resulted in the production of the Alpine chains as -we now see them. - -[Sidenote: VOLCANIC FISSURES OF PERMIAN PERIOD.] - -From the great fissures opened in Permian times along this line of -weakness, great quantities of lava, scoriæ, and tuff were poured out, -and these accumulated to form great volcanic mountains, which we can -now only study at a few isolated spots, as in the Tyrol, Carinthia, -and about Lake Lugano. Everywhere else, these Permian volcanic rocks -appear to be deeply buried under the later-formed sediments, from which -the Alpine chains have been carved. Few and imperfect, however, as are -the exposures of these ancient rhyolite and quartz-andesite lavas and -agglomerates formed at the close of the Palæozoic epoch, their greatly -denuded relics form masses which are in places more than 9,000 feet in -thickness. From this fact we are able to form some slight idea of the -scale upon which the volcanic outbursts in question must have taken -place during Permian times. - -The second stage in the series of operations by which the Alpine chains -have been formed, consisted in a general sinking of the surface along -that line of weakness in the earth's crust, the existence of which had -been betrayed by the formation of fissures and the eruption of volcanic -rocks. We have already had occasion to remark how frequently such -subsidences follow upon the extrusion of volcanic masses at any part -of the earth's surface; and we have referred these downward movements -in part to the removal of support from below the portion of the crust -affected, and in part to the weight of the materials piled upon its -surface by the volcanic forces. - -The volcanic energy which had been manifested with such violence -during the Permian period, does not appear to have died out altogether -during the succeeding Triassic period. A number of smaller volcanic -vents were opened from time to time, and from these, lavas, tuffs, and -agglomerates, chiefly of basic composition, were poured out. The relics -of these old Triassic volcanoes are found at many points along the -Alpine chain, but it is evident that the igneous forces were gradually -becoming exhausted during this period, and before the close of it they -had fallen into a state of complete extinction. - -But the great subsidence which had commenced in the Triassic period, -along what was to become the future line of the Alpine chain, was -continued almost without interruption during the Rhætic, the Jurassic, -the Tithonian, the Neocomian, the Cretaceous and the Nummulitic -periods. With respect to the strata formed during all these periods, -it is found that their thiknesses, which away from the Alpine axis -may be measured by hundreds of feet, is along that axis increased -to thousands of feet. The united thickness of sediments accumulated -along this great line of subsidence between the Permian and Nummulitic -periods probably exceeds 60,000 feet, or ten miles. The subsidence -appears to have been very slow and gradual, but almost uninterrupted, -and the deposition of sediments seems to have kept pace with the -sinking of the sea-bottom, a fact which is proved by the circumstance -that nearly the whole of these sediments were such as must have been -accumulated in comparatively shallow water. - -[Sidenote: FORMATION OF ALPINE GEOSYNCLINAL.] - -By the means we have described there was thus formed a 'geosynclinal,' -as geologists have called it, that is, a trough-like hollow filled -with masses of abnormally thickened sediments, which had been piled -one upon another during the long periods of time in which almost -uninterrupted subsidence was going on along the Alpine line of -weakness in the earth's crust. In this way was brought together that -enormous accumulation of materials from which the hard masses of the -Alpine chains were subsequently elaborated, and out of which the -mountain-peaks were eventually carved by denudation. - -The third stage in this grand work of mountain-making commenced in the -Oligocene period. It consisted of a series of movements affecting the -parts of the earth's crust on either side of the line of weakness -which had first exhibited itself in Permian times. By these movements -a series of tangential strains were produced, which resulted in the -violent crushing, folding, and crumpling of the sedimentary materials -composing the geosynclinal. - -One effect of this action was the violent flexure and frequent fracture -of these stratified masses, which are now found in the Alpine regions -assuming the most abnormal and unexpected positions and relations to -one another. Sometimes the strata are found tortured and twisted into -the most complicated folds and puckerings; at others they are seen to -be completely inverted, so that the older beds are found lying upon the -newer; and in others, again, great masses of strata have been traversed -by numerous fractures or faults, the rocks on either side of which are -displaced to the extent of thousands of feet. - -Another effect of the great lateral thrusts by which the thick -sedimentary masses of the geosynclinal were being so violently -disturbed, was the production of a great amount of induration and -chemical change in these rocks. Masses of soft clay, of the age of -that upon which London is built, were by violent pressure reduced to -the condition of roofing-slate, similar to that of North Wales. One -of the most important discoveries of modern times is that which has -resulted in the recognition of the fact of the mutual convertibility -of different kinds of energy. We now know that mechanical force -may be transformed into heat-force or chemical force; and of such -transformations we find abundant illustrations in the crushed and -crumpled rock-masses of the Alpine chains. - -Under the influence of these several kinds of force, not only was -extreme consolidation and induration produced among the rock-masses, -but chemical affinity and crystalline action had the fullest play -among the materials of which they were composed. In many cases we find -the originally soft muds, sands, and shell-banks converted into the -most highly crystalline rocks, which retain their primary chemical -composition, but have entirely lost all their other original features. - -[Sidenote: FORMATION OF ALPINE GEANTICLINAL.] - -To the mass of folded, crumpled, and altered strata, formed from a -geosynclinal by lateral pressure, geologists have given the name -of a 'geanticlinal.' The formation of the Alpine geanticlinal was -due to movements which commenced in the Oligocene period, attained -their maximum in the Miocene, and appear to have declined and almost -altogether died out in the Pliocene period. - -The movements which resulted in the crushing and crumpling of the -thickened mass of sediments along the Alpine line of weakness, also -gave rise to the formation of a series of fissures from which volcanic -action took place. These fissures were not, however, formed along the -original line of weakness, for this had been strengthened and repaired -by the deposition of ten-miles' thickness of sediments upon it, but -along new fissures opened in directions parallel to the original lines -of weakness, and in areas where a much less considerable amount of -deposition had taken place since Permian times. - -We have abundant evidence that, just at the period when those great -movements were commencing which resulted in the formation of the great -Alpine and Himalayan geanticlinal, earth-fissures were being opened -upon either side of the latter from which volcanic outbursts took -place. At the period when the most violent mountain-forming movements -occurred, these fissures were in their most active condition, and at -this time two great volcanic belts stretched east and west, on either -side of, and parallel to, the great Alpine chain. The Northern volcanic -band was formed by the numerous vents, now all extinct, in Auvergne, -Central Germany, Bohemia, and Hungary, and was probably continued in -the volcanoes of the Thian Shan and Mantchouria. The Southern volcanic -band was formed by the numerous vents of the Iberian and Italian -peninsulas, and the islands of the Mediterranean, and were continued -to the eastward by those of Asia Minor, Arabia, and the North Indian -Ocean. As the earth-movements which produced the geanticlinal died -away, the volcanic energy along these parallel volcanic bands died -away at the same time. In studying the geology of Central and Southern -Europe, no fact comes out more strikingly than that of the synchronism -between the earth-movements by which the geanticlinal of the Alps was -formed, and the volcanic manifestations which were exhibited along -lines of fissure parallel to that geanticlinal. The earth-movements -and the volcanic outbursts both commenced in the Oligocene period, -gradually attained their maximum in the Miocene, and as slowly declined -in the Pliocene. - -[Sidenote: SCULPTURING OF ALPS BY DENUDATION.] - -The fourth stage in the great work of mountain-building in the case -of the Alps consisted in the operation of the denuding forces, the -disintegrating action of rain and frost, the transporting action -of rivers and glaciers, by which the Alpine peaks were gradually -sculptured out of the indurated and altered masses constituting the -geanticlinal. The action of this fourth stage went on to a great -extent side by side with that of the third stage. So soon as the -earth-movements had brought the submerged sedimentary masses of the -geosynclinal under the action of the surface tides and currents of the -ocean, marine denudation would commence; and, as the work of elevation -went on, the rock-masses would gradually be brought within the reach -of those more silently-working but far more effective agents which -are ever operating in the higher regions of the atmosphere. It is -impossible to say what would have been the height of the Alpine chain -if the work of denudation had not to a great extent kept pace with -that of elevation. Only the harder and more crystalline masses have -for the most part escaped destruction, and stand up in high craggy -summits; while flanking hills, like the well-known Rigi, are Been to -be composed of conglomerates thousands of feet in thickness, composed -of their disintegrated materials. It is a remarkable fact, as showing -how enormous was the work of elevation daring the formation of the -geanticlinal, that some of the youngest and least consolidated rocks of -the Nummulitic period are still found at a height of 11,000 feet in the -Alps, and of 16,000 feet in the Himalaya. - -From what has been said, it will be seen that mountain-chains may be -regarded as cicatrised wounds in the earth's solid crust. A line of -weakness first betrays itself at a certain part of the earth's surface -by fissures, from which volcanic outbursts take place; and thus the -position of the future mountain-chain is determined. Next, subsidence -during many millions of years permits of the accumulation of the raw -materials out of which the mountain-range is to be formed; subsequent -earth-movements cause these raw materials to be elaborated into the -hardest and most crystalline rock-masses, and place them in elevated -and favourable positions; and lastly, denudation sculptures from these -hardened rock-masses all the varied mountain forms. Thus the work of -mountain-making is not, as was formerly supposed by geologists, the -result of a simple upheaving force, but is the outcome of a long and -complicated series of operations. - -[Sidenote: ORIGIN OF OTHER MOUNTAIN-CHAINS.] - -The careful study of other mountain-chains, especially those of the -American continent, has shown that the series of actions which we -have described as occurring in the Alps, took place in the same order -in the formation of all mountain-masses. It is doubtful whether the -line of weakness is always betrayed in the first instance by the -formation along its course of volcanic fissures. But in all cases we -have evidence of the production of a geosynclinal, which is afterwards, -by lateral pressure, converted into a geanticlinal, and from this the -mountain-chains have been carved by denudation. Professor Dana has -shown that the geosynclinal of the Appalachian chain was made up of -sediments attaining a thickness of 40,000 feet, or eight miles; while -Mr. Clarence King has shown that a part of the geosynclinal of the -Rocky Mountains was built up of no less than 60,000 feet, or twelve -miles of strata. - -It has thus been established that a very remarkable relation exists -between the forces by which continental masses of land are raised -and depressed, and mountain-ranges have been developed along lines -of weakness separating such moving continental masses, and those -more sudden and striking manifestations of energy which give rise to -volcanic phenomena. It is in this relation between the widespread -subterranean energies and the local development of the same forces -at volcanic vents, that we must in all probability seek for the -explanation of those interesting peculiarities of the distribution -of volcanoes upon the face of the globe which we have described -in a former chapter. The parallelism of volcanic bands to great -mountain-chains is thus easily accounted for; and in the same way -we may probably explain the position of most volcanoes with regard -to coast-lines. We have already pointed out the objections to the -commonly-received view that volcanoes depend for their supplies of -water on the proximity of the ocean. This proximity of the ocean to -volcanic vents we are thus inclined to regard, not as the cause, but as -the effect of the subterranean action. The positions of both volcanoes -and coast-lines are determined by the limits of those great areas of -the earth's crust which are subjected to slow vertical movements, often -in opposite directions. - -Terrible and striking, then, as are the phenomena connected with -volcanic action, such sudden and violent manifestations of the -subterranean energy must not be regarded as the only, or indeed the -chief, effects which they produce. The internal forces continually -at work within the earth's crust perform a series of most important -functions in connection with the economy of the globe, and were the -action of these forces to die out, our planet would soon cease to be -fit for the habitation of living beings. - -There is no fact which the geological student is more constantly -called upon to bear in mind than that of the potency of seemingly -insignificant causes which continue in constant operation through long -periods of time. Indeed these small and almost unnoticed agencies at -work upon the earth's crust are often found, in the long ran, to -produce far grander effects than those of which the action is much -more striking and obvious. It is to the silent and imperceptible -action of atmospheric moisture and frost that the disintegration of -the solid rock-masses must be mainly ascribed; and the noisy cataract -and ocean-billow produce effects which are quite insignificant -compared with those which must be ascribed to the slight and almost -unnoticed forces. Great masses of limestone are built up of the remains -of microscopic organisms, while the larger and higher life-forms -contribute but little to the great work of rock-building. - -[Sidenote: EFFECTS OF SLOW CONTINENTAL MOVEMENTS.] - -In the same way it is to the almost unnoticed action of the -subterranean forces in raising some vast areas of the earth's crust, -in depressing others, and in bringing about the development of -mountain-chains between them, that we must ascribe a far more important -part in the economy of our globe than to the more conspicuous but less -constant action of volcanoes. - -A few simple considerations will serve to convince us, not only of the -beneficial effects of the action of the subterranean energies within -the earth's crust, but of the absolute necessity of the continued -operation of those energies to the perpetuation of that set of -conditions by which our planet is fitted to be the habitation of living -beings. - -We have already referred to the prodigious effects which are constantly -being produced around us by the action of the external forces at work -upon the globe. The source of these external forces is found in the -movements and changes which are ever going on within the aqueous and -atmospheric media in which the globe is enveloped. The circulation of -the air, influencing the circulation of the waters in the shape of -clouds, rain, snow, rivers, glaciers, and oceans, causes the breaking -up of even the hardest rock-masses, and the continual removal of their -disintegrated fragments from higher to lower levels. This work goes on -with more or less regularity over every part of the land raised above -the level of the ocean, but the rate of destruction in the higher -regions of the atmosphere is far more rapid than at lower levels. Hence -the circulating air and water of the globe are found to be continually -acting as levellers of the land-masses of the earth. - -It is by no means a difficult task to calculate the approximate rate -at which the various continents and islands are being levelled down, -and such calculations prove that in a very few millions of years the -existing forces operating upon the earth's surface would reduce the -whole of the land-masses to the level of the ocean. - -But a little consideration will convince us that the circulation of -the air and waters of the globe are themselves dependent upon the -existence of those irregularities of the land-surfaces which they are -constantly tending to destroy. Without elevated mountain ridges the -regular condensation of moisture, and its collection and distribution -in streams and rivers over every part of the land surfaces, could -not take place. Under these circumstances the unchecked evaporation -of the oceanic waters would probably go on, till the proportion -of water-vapour increased to such an extent in the atmosphere as -effectually to destroy those nicely-balanced conditions upon which the -continued existence of both vegetable and animal life depend. - -But the repeated upward and downward movements which have been shown -to be going on in the great land-masses of the globe, giving rise -in turns to those lateral thrusts and tangential strains to which -mountain-chains owe their formation, afford a perfect compensation -to the action of the external forces ever operating upon the earth's -surface. - -If, however, the uncompensated effect of the external forces acting on -the earth's crust is calculated to bring about the destruction of those -conditions upon which the existence of life depends, the uncompensated -effect of the internal forces acting on the earth's crust are fraught -with at least equal dangers to those necessary conditions. - -[Sidenote: CONTRAST BETWEEN THE EARTH AND MOON.] - -In our nearest neighbour among the planets--the moon--the telescope has -revealed to us the existence of a globe, in which the internal forces -have not been checked and controlled by the operation of any external -agencies--for the moon appears to be destitute of both atmosphere and -water. - -Under these circumstances we find its surface, as we might expect, to -be composed of rocks which appear to be entirely of igneous origin; -the mountain-masses, unworn by rain or frost, river or glacier, being -of most prodigious dimensions as compared with those of our own globe, -while no features at all resembling valleys, or plains, or alluvial -flats are anywhere to be discerned upon the lunar surface. - -But by the admirable balancing of the external and internal forces -on our own globe, the conditions necessary to animal and vegetable -existence are almost constantly maintained, and those interruptions -of such conditions, produced by hurricanes and floods, by volcanic -outbursts and earthquakes, may safely be regarded as the insignificant -accidents of what is, on the whole, a very perfectly working piece of -machinery. - -The ancients loved to liken the earth to a living being--the macrocosm -of which man was the puny representative or microcosm; and when we -study the well-adapted interplay of the forces at work upon the -earth's crust, both from within and without, the analogy seems a -scarcely strained one. In the macrocosm and the microcosm alike, slight -interferences with the regular functions occasionally take place, and -both of them exhibit the traces of a past evolution and the germs of an -eventual decay. - - - - -CHAPTER XI. - -WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF THE EARTH'S INTERIOR. - - -In entering upon any speculations or enquiries concerning the nature of -the interior of our globe, it is necessary before all things that we -should clearly realise in our minds how small and almost infinitesimal -is that part of the earth's mass which can be subjected to direct -examination. The distance from the surface to the centre of our globe -is nearly 4,000 miles, but the deepest mines do not penetrate to much -more than half a mile from the surface, and the deepest borings fall -far short of a mile in depth. Sometimes, it is true, the geologist -finds means for drawing inferences as to the nature of the rocks at -depths of ten or fifteen miles below the surface; but the last-named -depth must be regarded as the utmost limit of that portion of our -globe which can be made the object of direct observation and study. -This thin exterior film of the earth's mass, which the geologist is -able to investigate, we call the 'crust of the globe'; but it must be -remembered that in using this term, it is not intended to imply that -the outer part of our globe differs in any essential respect from the -interior. The term 'crust of the globe' is employed by geologists as -a convenient way of referring to that portion of the earth which is -accessible to their observation. - -But if we are unable to make direct investigations concerning the -nature of the internal portions of the globe, there are nevertheless a -number of facts from which we may draw important inferences upon the -subject. These facts and the inferences based upon them we shall now -proceed to consider. - -First in importance among these we may mention the results which have -been obtained by weighing our globe. Various methods have been devised -for accomplishing this important object, and the conclusions arrived -at by different methods agree so closely with one another, that there -is no room for doubt as to the substantial accuracy of those results. -It may be taken as proved beyond the possibility of controversy that -our globe is equal in weight to five and a half globes of the same size -composed of water, or, in other words, that the average density of the -materials composing the globe is five and a half times as great as that -of water. - -Now the density of the materials which compose the crust of the globe -is very much less than this, varying from about two-and-one-third to -three times that of water. Hence we are compelled to conclude that the -interior portions of the globe are of far greater density than the -exterior portions; that, as a matter of fact, the mass of the globe is -composed of materials having twice the density of the rocks exposed at -the surface. - -[Sidenote: DENSITY OF EARTH'S INTERIOR.] - -It has been sometimes argued that as all materials under intense -pressure appear to yield to an appreciable extent, and to allow their -particles to be packed into a smaller compass, we may find in this fact -an explanation of the great density of the internal parts of the globe. -It has in fact been suggested that under the enormous pressure which -must be exerted by masses of rock several thousand feet in thickness, -the materials of which our earth is composed may be compelled to pack -themselves into less than one-half the compass which they occupy at the -surface. But the ascription of such almost unlimited compressibility to -solid substances can be supported neither by experiment nor analogy. -Various considerations point to the probability that solid bodies yield -to pressure up to a certain limit and no farther, and that when this -limit is reached an increase in pressure is no longer attended with a -reduction in bulk. - -If then we are compelled to reject the idea of the unlimited -compressibility of solid substances, we must conclude that the interior -portions of our globe are composed of _materials of a different kind_ -from those which occur in its crust. And this conclusion, as we shall -presently see, is borne out by a number of independent facts. - -The study of the materials ejected from volcanic vents proves that even -at very moderate depths there exist substances differing greatly in -density, as well as in chemical composition. The lightest lavas have a -specific gravity of 2·3, the heaviest of over 3. And that materials of -even greater density are sometimes brought by volcanic action from the -earth's interior, we have now the clearest proofs. - -[Sidenote: RELATION BETWEEN EARTH AND OTHER PLANETS.] - -But in considering a question of this kind, it will be well to remember -that analogy may furnish us with hints upon the subject which may prove -to be by no means unimportant. There is no question upon which modern -science has wrought out a more complete revolution in our ideas, than -that of the relation of our earth to the other bodies of the universe. -We know, as the result of recent research, that our globe is one of -a great family of bodies, moving through space in similar paths and -in obedience to the same laws. A hundred years ago the primary and -secondary planets of the solar system could be almost numbered upon -the fingers; now we recognise the fact that they exist in countless -millions, presenting every variety of bulk from masses 1,400 times -as large as our earth down to the merest planetary dust. Between the -orbits of Mars and Jupiter, more than 200 small planets have been -recognised as occurring, and every year additions are made to the -number of these asteroids. Comets have now been identified with streams -of such planetary bodies, of minute size, moving in regular orbits -through our system. The magnificent showers of 'shooting-stars' have -been proved to be caused by the passage of the earth through such -bands of travelling bodies, and 'the zodiacal light' finds its most -probable explanation in the supposition that the sun is surrounded by -a great mass of such minute planets. Every increase in the power of -the telescope reveals to us the existence of new secondary planets -or moons, revolving about the primaries; and the wonderful system -of the Saturnian rings is now explained by the proved existence of -great streams of such secondary planets circling around it. The solar -system was formerly conceived of as a vast solitude through which a -few gigantic bodies moved at awful distances from one another. Now we -know that the supposed empty void is traversed by countless myriads of -bodies of the most varied dimensions, all moving in certain definite -paths, in obedience to the same laws, ever acting and reacting upon -each other, and occasionally coming into collision. - -There are not wanting further facts to prove that the other planets are -like our own in many of their phenomena and surroundings. In some of -them atmospheric phenomena have been detected, such as the formation of -clouds and the deposition of snow, so that the external forces at work -on our globe act upon them also. And that internal forces, like those -we have been considering in the case of our earth, are at work in our -neighbours, is proved by the great solar storms and the condition of -the moon's surface. - -But the results of spectrum-analysis in recent years have furnished new -facts in proof of the close relationship of our earth to the numerous -similar bodies by which it is surrounded. So far as observation has yet -gone we have reason for believing that not only the members of the -solar system, but the more distant bodies of the universe, are all -composed of the same elementary substances as those which enter into -the composition of our globe. - -The most satisfactory information concerning the composition and nature -of other planetary bodies is derived from the study of those small -planets which occasionally come into collision with our globe, and -which have their own proper motion in space thereby arrested. These -meteorites, as such falling planetary bodies are called, have justly -attracted great attention, and their fragments are treasured as the -most valuable objects in our museums. - -[Sidenote: COMPOSITION OF METEORITES.] - -The first fact concerning these meteorites, which it is necessary -to notice, is that they are composed of the same chemical elements -as occur in the earth's crust. No element has yet been found in any -meteorite which was not previously known as existing in the earth, and -of the sixty-five or seventy known terrestrial elements no less than -twenty-two have already been detected in meteorites. - -There are, however, a dozen elements which occur in overwhelming -proportions in the earth's crust. We shall probably not be going too -far in saying that these twelve elements--namely, oxygen, silicon, -aluminium, calcium, magnesium, sodium, potassium, iron, carbon, -hydrogen, sulphur, and chlorine--make up amongst them not less than 999 -out of 1,000 parts of the earth's crust, and that all the other fifty -or sixty elements are 80 comparatively rare that they do not constitute -when taken altogether more than one part in 1,000 of the rocks of the -globe. Now all of these twelve common terrestrial elements occur in -meteorites, and the fact that the rarer terrestrial elements have not -as yet been found in them will not surprise anyone, who remembers how -small is the bulk of all the specimens of these meteorites existing in -our museums. - -We have hitherto insisted on the points of resemblance in the chemical -composition of meteorites and that of the rocks of the globe, but we -shall now have to indicate some very important points in which they -differ. - -While in the rocks composing the earth's crust oxygen forms one-half of -their mass, and silicon another quarter, we find that in the meteorites -these elements, though present, play a much less important part. The -most abundant element in the meteorites is iron; and nickel, chromium, -cobalt, manganese, sulphur, and phosphorus, are much more abundant in -these extra-terrestrial bodies than they are in the earth's crust. - -We have already referred to the remarkable fact that in our earth's -crust nearly all the other elementary substances are found combined -in the first instance with oxygen, and that most rocks consist of the -oxide of silicon combined with the oxides of various metals. But this -is by no means the case with the meteorites. In them we find metals -like iron, nickel, cobalt, &c., in their uncombined condition, and -forming alloys with one another. The same and other metals also occur -in combination with carbon, phosphorus, chlorine, and sulphur, and -some of the substances thus formed are quite unknown among terrestrial -rocks. Compounds of the oxide of silicon with the oxides of the metals -such as form the mass of the crust of the globe do occur in meteorites, -but they play a much less important part than in the case of the -terrestrial rocks. - -Among the substances found in meteorites are several which do not exist -among the terrestrial rocks--some, indeed, which it seems impossible -to conceive of as being formed and preserved under terrestrial -conditions. Among these we may mention the phosphide of iron and nickel -(Schreibersite), the sulphide of chromium and iron (Daubréelite), the -protosulphide of iron (Troilite), the sulphide of calcium (Oldhamite), -the protochloride of iron (Lawrencite), and a peculiar form of -crystallised silica, called by Professor Maskelyne 'Asmanite.' - -[Sidenote: DIFFERENT KINDS OF METEORITES.] - -There are other phenomena exhibited by meteorites which indicate that -they must have been formed under conditions very different to those -which prevail upon the earth's surface. Thus we find that fused iron -and molten slag-like materials have remained entangled with each other, -and have not separated as they would do if a great body like the earth -were near to exercise the varying force of gravity upon the two -classes of substances. Again, meteorites are found to have absorbed -many times their bulk of hydrogen gas, and to exhibit peculiarities in -their microscopic structure which can probably be only accounted for -when we remember that they were formed in the interplanetary spaces, -far away from any great attracting body. - -But in recent years a number of very important facts have been -discovered which may well lead us to devote a closer attention to the -composition and structure of meteorites. It has been shown, on the -one hand, that some meteorites contain substances precisely similar -to those which are sometimes brought from the earth's interior during -volcanic outbursts; and, on the other hand, there have been detected, -among some of the ejections of volcanoes, bodies which so closely -resemble meteorites that they were long mistaken for them. Both kinds -of observation seem to point to the conclusion that the earth's -interior is composed of similar materials to those which we find in the -small planets called meteorites. - -M. Daubrée has proposed a very convenient classification for -meteorites, dividing them into the following four groups:-- - -I. _Holosiderites_; consisting almost entirely of metallic iron, or of -iron alloyed with nickel, stony matter being absent; but sulphides, -phosphides, and carbides of several metals are often diffused through -the mass. The polished surfaces of these meteoric irons, when etched -with acid, often exhibit a remarkable crystalline structure. - -II. _Syssiderites_; in which a network of metallic iron encloses a -number of granular masses of stony materials. - -III. _Sporadosiderites_; which consist of a mass of stony materials, -through which particles of metallic iron are disseminated. - -IV. _Asiderites_; containing no metallic iron, but consisting entirely -of stony materials. - -There are, besides the meteorites belonging to these principal groups, -a few of peculiar and exceptional composition, which we need not notice -further for our present purpose. - -From the above classification it will be seen that most meteorites -consist of a mixture in varying proportions of metallic and stony -materials. Sometimes the metallic constituents are present in greater -proportions than the stony, at other times the stony materials -predominate, while occasionally one or other of these elements may be -wholly wanting. - -The stony portions of meteorites, upon careful examination, prove to be -built up of certain minerals, agreeing in their chemical composition -and their crystalline forms with those which occur in the rocks of the -earth's crust. Among the ordinary terrestrial minerals occurring in -the stony portions of meteorites, we may especially mention olivine, -enstatite, augite, anorthite, chromite, magnetite, and pyrrhotite. - -[Sidenote: METEORITES AND ULTRA-BASIC ROCKS.] - -The minerals which occur in meteorites are in every case such as are -found in the more basic volcanic rocks--quartz, and the acid felspars, -with the other minerals which occur in acid rocks, being entirely -absent in the 'extra-terrestrial' rocks. - -Now, besides the three great classes of lavas which we have described -as being ejected from volcanic vents, there are some rarer materials -occasionally brought from the earth's interior by the same agency, -that present a most wonderful resemblance to the stony portions of -meteorites. These materials we may call 'ultra-basic rocks.' Their -specific gravity is very high, usually exceeding 3, and they contain -a very low percentage of silica; on the other hand, the proportion of -iron and magnesia is often much greater than in ordinary terrestrial -rocks. But the most remarkable fact about these ultra-basic rocks is, -that they are almost entirely composed of the minerals which occur in -meteorites; namely, olivine, enstatite, augite, anorthite, magnetite, -and chromite. - -The ultra-basic rocks often occur under very peculiar conditions. -Sometimes they are found forming ordinary volcanic protrusions -through the sedimentary rocks. The rocks named pikrites, lherzolites, -dunites, &c., are examples of such igneous protrusions composed of -these ultra-basic materials, and probably all the true serpentines are -rocks of the same class which have absorbed water and undergone great -alteration. The ultra-basic rocks sometimes contain platinum and other -metals in the free or uncombined state. But not unfrequently we find -among the ordinary ejections of volcanoes, nodules and fragments of -such ultra-basic materials, which have clearly been carried up with the -other lavas from great depths in the earth's crust. Thus in Auvergne, -the Eifel, Bohemia, Styria, and many other volcanic districts, the -basaltic lavas and tuffs are found to contain nodules composed of -the minerals which are so highly characteristic of meteorites. Such -nodules, too, often form the centres of the volcanic bombs which are -thrown out of craters during eruptions. - -We thus see that materials identical in composition and character with -the stony portions of meteorites, exist within the earth's interior, -and are thrown out on its surface by volcanic action. A still more -interesting discovery has been made in recent years; namely, that -materials similar to the metallic portion of meteorites, and consisting -of nickeliferous iron, also occur in deep-seated portions of the -earth's crust, and are brought to the surface during periods of igneous -activity. - -In the year 1870, Professor Nordenskiöld made a most important -discovery at Ovifak, on the south side of the Island of Disko, off -the Greenland coast. On the shore of the island a number of blocks of -iron were seen, and the chemical examination of these proved that, -like ordinary metallic meteorites, they consisted of iron alloyed with -nickel and cobalt. - -[Sidenote: IRON-MASSES OF OVIFAK.] - -Now, when the facts concerning the masses of native iron of Ovifak were -made known, the first and most natural explanation which presented -itself to every mind was, that these were a number of meteorites which -at some past period had fallen upon the earth's surface. - - Metallic iron. - - Opaque crystals of magnetite (black oxide of iron). - - Transparent crystals of felspar, augite, and olivine. - -[Illustration: Fig. 87.--Section of basalt from Ovifak, Greenland, with -particles of metallic iron diffused through its mass.] - -But a further examination of the locality revealed a number of facts -which, as Professor Steenstrup pointed out, it is very difficult -to reconcile with the theory that the Ovifak masses of iron are of -meteoric origin. The district of Western Greenland, where these masses -were discovered, has been the scene of volcanic outbursts on the -grandest scale during the Miocene period. In close proximity to the -great iron masses, there are seen a number of basaltic dykes; and, when -these dykes are carefully examined, the basaltic rock of which they are -composed is seen to be full of particles of metallic iron. In fig. 87, -we have a drawing made from a section of the Ovifak basalts magnified -four or five diameters. The rock-mass is seen to be composed of black, -opaque magnetite, and transparent crystals of augite, labradorite, -olivine, &c.; while, through the whole, particles of metallic iron are -found entangled among the different crystals in the most remarkable -manner. - -It has been suggested that this singular rock might have been formed by -a meteorite falling, in Miocene times, into a lava-stream in a state -of incandescence. But the relation of the metallic particles to the -stony materials is such as to lend no support whatever to this rather -strained hypothesis. - -A careful study of all the facts of the case by Lawrence Smith, -Daubrée, and others well acquainted with the phenomena exhibited by -meteorites, has led to the conclusion that the large iron-masses of -Ovifak, as well as the particles of metallic iron diffused through -the surrounding basalts, are all of terrestrial origin, and have been -brought by volcanic action from the earth's interior. It is probable -that, just as we find in many basaltic lavas nodules of ultra-basic -materials similar to the stony parts of meteorites, so in these basalts -of Ovifak we have masses of iron alloyed with nickel, similar to the -metallic portions of meteorites. Both the stony and metallic enclosures -in the basalt are in all probability derived from deeper portions of -the earth's crust. By the weathering away of the basalt of Ovifak, the -larger masses of metallic iron have been left exposed upon the shore -where they were found. - -There are a number of other facts which seem to support this startling -conclusion. Thus it has been shown by Professor Andrews that certain -basalts in our own islands contain particles of metallic iron of -microscopic dimensions, and it is not improbable that some of the -masses of nickeliferous iron found in various parts of the earth's -surface, which have hitherto been regarded as meteorites, are, like -those of Ovifak, of terrestrial origin. - -[Sidenote: MATERIALS FILLING METALLIC-VEINS.] - -Another piece of evidence pointing in the same direction, is derived -from those great fissures communicating with the interior of our globe -which become filled with metallic minerals, and are known to us as -mineral-veins. In these mineral-veins the native metals, their alloys, -and combinations of these with sulphur, chlorine, phosphorus, &c., are -frequently present. But oxides of the metals, except as products of -subsequent alteration, occur far less frequently than in the earth's -crust generally. Hence we are led to conclude that the substances which -in the outer part of the earth's crust always exist in combination -with oxygen, are at greater depths in a free and uncombined condition. - -Nor is it a circumstance altogether unworthy of attention that the -researches of Mr. Norman Lockyer and other astronomers, based on the -known facts of the relative densities of the several members of the -solar system, and the ascertained relations of the different solar -envelopes, have led to conclusions closely in accord with those arrived -at by geologists. These researches appear to warrant the hypothesis -that the interior of our globe consists of metallic substances -uncombined with oxygen, and that among these metallic substances iron -plays an important part. Our globe, as we know, is a great magnet, -and the remarkable phenomena of terrestrial magnetism may also not -improbably find their explanation in the fact that metallic iron forms -80 large a portion of the earth's interior. - -The interesting facts which we have been considering may be made -clearer by the accompanying diagram (fig. 88). The materials ejected -from volcanic vents (lavas) are in almost all cases compounds of -silicon and the various metals with oxygen. In the lighter or acid -lavas oxygen constitutes one-half of their weight, and the proportion -of metals of the iron-group is very small. As we pass to the heavier -intermediate and basic lavas, we find the proportion of oxygen -diminishing, and the metals of the alkaline earths (magnesium and -calcium) with the metals of the iron-group increasing, in quantity. In -the small and interesting group of the ultra-basic lavas the proportion -of oxygen is comparatively small, and the proportion of magnesium and -iron very high. So much for the terrestrial rocks. - -[Illustration: Fig. 88.--Diagram illustrating the relation between the -Terrestrial and the Extra-Terrestrial Rock.] - -[Sidenote: TERRESTRIAL AND EXTRA-TERRESTRIAL ROCKS.] - -Now let us turn our attention to the extra-terrestrial rocks or those -found in meteorites. The Asiderites are quite identical in composition -with the ultra-basic lavas of our globe, but in the Sporadosiderites -and the Syssiderites we find the proportion of oxygen rapidly -diminishing, and that of metallic iron increasing. Finally, in the -Holosiderites the oxygen entirely disappears, and the whole mass -becomes metallic. - -From the Holosiderites at one end of the chain to the add lavas -at the other, we find there is a complete and continuous series; -the rocks of terrestrial origin overlapping, in their least -oxydized representatives, the most highly oxydized representatives -of the extra-terrestrial rocks. But the discovery at Ovifak of -the iron-masses, and the basalts with iron disseminated, has -afforded another very important link, placing the terrestrial and -extra-terrestrial rocks in closer relations with one another. - -All these facts appear to point to the conclusion that the earth's -interior consists of metallic substances either quite uncombined -or simply alloyed with one another, and among these iron is very -conspicuous by its abundance. The outer crust, which is probably of no -great thickness, contains an enormous proportion of oxygen and silicon -combined with the materials which constitute the interior portions of -our globe. It may be, as has been suggested by astronomers, that our -earth consisted at one time of a solid metallic mass surrounded by a -vaporous envelope of metalloids, and that the whole of the latter, with -the exception of the constituents of the atmosphere and ocean, have -gradually entered into combination with the metals of the nucleus to -form the existing crust of the globe. But of this period the geologist -can take no cognisance. The records which he studies evidently -commenced at a long subsequent period, when the conditions prevailing -at the earth's surface differed but little, if at all, from those which -exist at the present day. Equally little has the geologist to do with -speculations concerning a far distant future when, as some philosophers -have suggested, the work of combination of the waters and atmosphere of -the earth's surface with the metallic substances of its interior shall -be completed, and our globe, entirely deprived of its fluid envelopes, -reduced to the condition in which we find our satellite, the moon. - - * * * * * - -[Sidenote: PHYSICAL CONDITION OF EARTH'S INTERIOR.] - -There is another class of enquiries concerning the earth's interior to -which the attention of both geologists and astronomers has long been -directed--that, namely, which deals with the problem of the _physical -condition_ of the interior of our globe. - -The fact that masses of molten materials are seen at many points of the -earth's surface to issue from figures in the crust of our globe, seems -at first sight to find a simple explanation if we suppose our planet -to consist of a fluid central mass surrounded by a solid crust. Hence -we find that among those who first thought upon this subject, this -hypothesis of a liquid centre and a solid crust was almost universally -accepted. This hypothesis was supposed to find further support in the -fact that, as we penetrate into the earth's crust by mines or boring -operations, the temperature is found to continually increase. It was -imagined, too, that this condition of our planet would best agree with -the requirements of the nebular hypothesis of Laplace, which explains -the formations and movements of the bodies of the solar system by the -cooling down of a nebulous mass. - -But a more careful and critical examination of the question has led -many geologists and astronomers to reject the hypothesis that the earth -consists of a great fluid mass surrounded by a comparatively thin shell -of solid materials. - -Volcanic outbursts and earthquake tremors, though so terrible and -destructive to man and his works, are but slight and inconsiderable -disturbances in a globe of such vast dimensions as that on which we -live. The condition of the crust of the globe is, in spite of volcanic -and earthquake manifestations, one of general stability; and this -general stability has certainly been maintained during the vast periods -covered by the geological record. Such a state of things seems quite -irreconcilable with the supposition that, at no great depth from the -surface, the whole mass of the globe is in a liquid condition. If, on -the other hand, it be supposed that the solid crust of the globe is -several hundreds of miles in thickness, it is difficult to understand -how the local centres of volcanic activity could be supplied from such -deep-seated sources. - -There are other facts which seem equally irreconcilable with the -hypothesis of a fluid centre and a thin solid crust in our globe. If -all igneous products were derived from one central reservoir, we might -fairly expect to find a much greater uniformity of character among -those products than really exists. But in some cases, materials of -totally different composition are ejected at the same time from closely -adjoining volcanic districts. Thus in Hungary and Bohemia, as we have -seen, lavas of totally different character were being extruded during -the Miocene period. In the island of Hawaii, as Professor Dana has -pointed out, igneous ejections have taken place at a crater 14,000 feet -above the sea-level, while a closely adjoining open vent at a level -10,000 feet lower exhibited no kind of sympathy with the disturbance. -Whatever may be the cause of volcanic action, it seems clear that it -does not originate in a universal mass of liquefied material situated -at no great depth from the earth's surface. - -The conclusions arrived at by astronomers and physicists is one quite -in accord with those which geologists have reached by totally different -methods. It is now very generally admitted that if the earth were not -a rigid mass, its behaviour under the attract live influences of the -surrounding members of the solar system would be very different to what -is found to be the case. - -[Sidenote: ARGUMENTS AGAINST LIQUID INTERIOR.] - -That the earth is in a solid condition to a great depth from the -surface, and possibly quite to the centre, is a conclusion concerning -which there can be little doubt; and in the next chapter we shall -endeavour to show that such a condition of thirds is by no means -incompatible with those manifestations of internal energy, the -phenomena of which we are considering in this work. The question, -therefore, of the complete solidity of our globe, or of its consisting -of a solid and a liquid portion, is one of speculative interest only, -and is in no way involved in our investigations concerning the nature -and origin of volcanic activity. We may conclude this chapter by -enumerating the several hypotheses which have at different times been -maintained concerning the nature of the interior of our globe. - -_First._ It has been suggested that the earth consists of a fluid or -semi-fluid nucleus surrounded and enclosed in a solid shell. Some -have maintained this shell to be of such insignificant thickness, as -compared with the bulk of the interior liquid mass, that portions of -the latter are able to reach the earth's surface through movements -and fractures of the outer shell, and that in this manner volcanic -manifestations originate. Others, impressed with the general stability -and rigidity of the globe as a whole, have maintained that the outer -solid shell must have a very considerable thickness, amounting -probably to not less than several hundreds of miles. But through a -shell of such thickness it is difficult to conceive of the liquid -masses of the interior finding their way to the surface, and those who -have held this view are driven to suggest some other means by which -local developments of volcanic action might be brought about. - -_Secondly._ Some physicists have asserted that a globe of liquid -matter radiating its heat into space, would tend to solidify both at -the surface and the centre, at the same time. The consequence of this -action would be the production of a sphere with a solid external shell -and a solid central nucleus, but with an interposed layer in a fluid -or semi-fluid condition. It has been pointed out that if we suppose -the solidification to have gone so far, as to have caused the partial -union of the interior nucleus and the external shell, we may conceive a -condition of things in which the stability and rigidity is sufficient -to satisfy both geologists and astronomers, but that in still -unsolidified pockets or reservoirs, filled with liquefied rock, between -the nucleus and the shell, we should have a competent cause for the -production of the volcanic phenomena of the globe. In this hypothesis, -however, it is assumed that the cooling at the centre and the surface -of the globe would go on at such rates that the reservoirs of liquid -material would be left at a moderate depth from the surface, so that -easy communication could be opened between them and volcanic vents. - -[Sidenote: REVIEW OF THE SEVERAL HYPOTHESES.] - -_Thirdly._ It has been maintained that the earth may have become -perfectly solid from the centre to the surface. Those who hold this -view endeavour to account for the phenomena of volcanoes in one of two -ways. It may be, they say, that the deep-seated rock-masses, though -actually solid, are in a state of _potential_ liquidity; that though -reduced to a solid state by the intense pressure of the superincumbent -masses, yet such is the condition of unstable equilibrium in the whole -mass, that the comparatively slight movements and changes taking place -at the earth's surface suffice to bring about the liquefaction of -portions of its crust and consequent manifestations of volcanic energy. -But It may be, as other supporters of the doctrine of the earth's -complete solidity have maintained, that the phenomena of volcanoes -have no direct connection with a supposed incandescent condition of -our planet at all, and that there are chemical and mechanical forces -at work within our globe which are quite competent to produce at the -surface all those remarkable phenomena which we identify with volcanic -action. - -From this summary of the speculative views which have been entertained -upon the subject of the physical condition of the earth's interior, -it will be clear that at present we have not sufficient evidence for -arriving at anything like a definite solution of the problem. The -conditions of temperature and pressure which exist in the interior of -a globe of such vast dimensions as our earth, are so far removed from -those which we can imitate in our experimental enquiries, and it is so -unsafe to push the application of laws arrived at by the latter to the -extreme limits required by the former, that we shall do well to pause -before attempting to dogmatise on such a difficult question. - -In the next chapter we shall endeavour to grapple with a somewhat more -hopeful task, to point out how far observation and experiment have -enabled us to offer a reasonable explanation of the wonderful series of -phenomena which are displayed during outbursts of volcanic activity. - - - - -CHAPTER XII. - -THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN THE CAUSES OF VOLCANIC -ACTION. - - -Every completed scientific investigation must consist of four series -of operations. In the first of these an attempt is made to collect the -whole of the facts bearing on the question, by means of observation -and experiment; the latter being only observation under conditions -determined by ourselves. In the second stage of the enquiry, the -attention is directed to classifying and grouping the isolated facts, -so as to determine their bearings upon one another, and the general -conclusions to which they appear to point. In the third stage, it is -sought to frame an hypothesis which shall embrace all the observed -facts, and shall be in harmony with the general conclusions derived -from them. In the fourth stage, this hypothesis is put to the most -rigid test; comparing the results which must follow, if it be true, -with the phenomena actually observed, and rejecting or amending our -hypothesis accordingly. Every great scientific theory has thus been -established by these four processes--observation, generalisation, -hypothesis, and verification. - -The enquiry concerning the nature and causes of volcanic action is far -from being a completed one. It is true that many hypotheses upon the -subject have been framed, but in too many instances these have not been -based on accurate observations and careful generalisations, and can be -regarded as little better than mere guesses. Indeed, the state of the -enquiry at the present time would seem to be as follows. Although much -remains to be done in the direction both of observation and experiment, -the main facts of the case have been established upon irrefragable -evidence. The classification and comparison of these facts have led to -the recognition of certain laws, which seem to embrace all the known -facts. To account for these facts and their demonstrated relations to -one another, certain tentative hypotheses have been suggested; but in -no case can it be truly said that these latter have so far stood the -test of exact enquiry as to deserve to rank as demonstrated truths. A -complete and consistent theory of volcanic action still remains to be -discovered. - -[Sidenote: VALUE AND LIMITS OF HYPOTHESES.] - -In accordance with the plan which we have sketched out for ourselves -at the commencement of this work, we shall aim at following what has -been the order of investigation and discovery in our study of volcanic -action; and in this concluding chapter we shall indicate the different -hypotheses by which it has been proposed to account for the varied -phenomena, which we have discussed in the preceding pages, and their -remarkable relations to one another. We shall endeavour, in passing, -to indicate how far these several hypotheses appear to be probable, -as satisfying a larger or smaller number of those conditions of the -problem which have been established by observation, experiment, and -careful reasoning; but we shall at the same time carefully avoid such -advocacy of any particular views as would tend to a prejudgment of -the question. Hypothesis is, as we have seen, one of the legitimate -and necessary operations in scientific investigation. It only becomes -a dangerous and treacherous weapon when it is made to precede rather -than to follow observation and experiment, or when being regarded -with paternal indulgence, an attempt is made to shield it from the -relentless logic of facts. Good and bad hypotheses must be allowed to -'grow together till the harvest;' such as are unable to accommodate -themselves to the surrounding conditions imposed by newly-discovered -facts and freshly-established laws will assuredly perish; and in this -'struggle for existence' the true hypothesis will in the end survive, -while the false ones perish. - -It may well happen, however, that among the hypotheses which have up to -the present time been framed, none will be found to entirely satisfy -all the conditions of the problem. New discoveries in physics and -chemistry have suggested fresh explanations of volcanic phenomena in -the past, and may continue to do so in the future; and the true theory -of volcanic action, when it is at last discovered, may combine many of -the principles which now seem to be peculiar to different hypotheses. - -Let us, in the first place, enquire what are the facts which must be -accounted for in any theory of volcanic action. We have already been -led to the conclusion that the phenomena exhibited by volcanoes were -entirely produced by the escape of imprisoned water and other gases -from masses of incandescent and fluid rock. Our subsequent examination -of the problem confirmed the conclusion that in all cases of volcanic -outburst we have molten rock-materials from which water and other -gases issue with greater or less violence. The two great facts to -be accounted for, then, in any attempted explanation of volcanic -phenomena, are the existence of this high temperature at certain points -within the earth's crust, and the presence of great quantities of water -and gas, imprisoned in the rocks. We shall perhaps simplify the enquiry -if we examine these two questions separately, and, in the first place, -review those hypotheses which have been suggested to account for high -temperatures in the subterranean regions, and, in the second place, -examine those which seek to explain the presence of large quantities of -imprisoned water and gases. - -[Sidenote: INCREASE OF TEMPERATURE WITH DEPTH.] - -That a high temperature exists in the earth's crust at some depth from -the surface is a £act which does not admit of any doubt. Every shaft -sunk for mining operations, and every deep boring made for the purpose -of obtaining water, proves that a more or less regular increase of -temperature takes place as we penetrate downwards. The average rate -of this increase of temperature has been estimated to be about 1° -Fahrenheit for every 50 or 60 feet of depth. - -Now if it be assumed that this regular increase of temperature -continues to great depths, a simple calculation proves that at a depth -of 9,000 feet a temperature of 212° Fahrenheit will be found--one -sufficient to boil water at the earth's surface--while at a depth of 28 -miles the temperature will be high enough to melt cast-iron, and at 34 -miles to fuse platinum. - -So marked is this steady increase of temperature as we go downwards, -that it has been seriously proposed to make very deep borings in order -to obtain supplies of warm water for heating our towns. Arago and -Walferdin suggested this method for warming the Jardin des Plantes at -Paris; and now that such important improvements have been devised in -carrying borings to enormous depths, the time may not be far distant -when we shall draw extensively upon these supplies of subterranean -heat. At the present time the city of Buda-Pesth is extensively -supplied with hot-water from an underground source. Should our -coal-supply ever fail it may be well to remember that we have these -inexhaustible supplies of heat everywhere beneath our feet. - -But although we may conclude that at the moderate depths we have -indicated such high temperatures exist, it would not be safe to -infer, as some have done, that at a distance of only 40 or 50 miles -from the surface the materials composing our globe are in a state -of actual fusion. Both theory and experiment indicate that under -increased pressure the fusing point of solid bodies is raised; and -just as in a Papin's digester we may have water retained by high -pressure in a liquid condition at a temperature far above 212° F., -so in the interior of the earth, masses of rock may exist in a solid -state, at a temperature far above that at which they would fuse at -the earth's surface. We may speak of such rock-masses, retained in a -solid condition by intense pressure, at a temperature far above their -fusing point at the earth's surface, as being in a 'potentially liquid -condition.' Upon any relief of pressure such masses would at once -assume the liquid state, just as the superheated water in a Papin's -digester immediately flashes into steam upon the fracture of the strong -vessel by which it is confined. We have already seen how the action -at volcanic vents often appears to indicate just such a manifestation -of elastic forces, as would be exhibited by the relief of superheated -masses from a state of confinement by pressure. - -In reasoning upon questions of this kind, however, we must always be -upon our guard against giving undue extension to principles and laws -which seem to be clearly established by experiment at the earth's -surface. It is well to remember how exceedingly limited is our command -of extreme pressures and high temperatures, when compared with those -which may exist within a body of the dimensions of our globe. - -[Sidenote: EFFECT OF PRESSURE ON FUSION-POINT.] - -If we were to imagine a set of intelligent creatures, who were able to -command only a range of temperatures from 50° to 200° F., engaged upon -an investigation of the properties of water, we shall easily understand -how unsafe it may be to extend generalisations far beyond the limits -covered by actual experiment. Such beings, from their observation of -the regular changes of volume of water at all the temperatures they -could command, might infer that at still higher and lower temperatures -the same rates of expansion and contraction would be maintained. Yet, -as we well know, such an inference would be quite wide of the truth; -for a little above 200° F. water suddenly expands to 1,700 times its -volume, and not far below 50° F. the contraction is suddenly changed -for expansion. - -It has been argued by the late Mr. David Forbes and others that, -inasmuch as experiment has shown that--though the fusing points of -solids are raised by pressure, yet that this rise of the fusing -points goes on in a diminishing ratio as compared with the pressures -applied--a limit will probably be reached at which the most intense -pressure will not be sufficient to retain substances at a high -temperature in their solid state. The fact that gases cannot be -retained in a liquid condition by the most intense pressure at a -temperature above their critical point, may seem by analogy to favour -the same conclusion. Hence, David Forbes, Dana, and other authors, have -argued in favour of the existence of a great liquid nucleus in our -globe covered by a comparatively thin, solid crust. And if we accept -the supposed proofs of a constant increase of temperature from the -surface to the centre of the globe, such a conclusion appears to be at -least as well founded as that which regards the central masses of the -earth as maintained in a solid condition by intense pressure. - -A little consideration will, however, convince us that the facts which -have been relied upon as proving the intensely heated condition of the -central masses of our globe, are by no means so conclusive as has been -supposed. - -The earth's form, which mathematicians have shown to be exactly that -which would be acquired by a globe composed of yielding materials -rotating on its axis at the rate which our planet does, has often -been adduced as proving that the latter was not always in a rigid and -unyielding condition. In the same way, all the remarkable facts and -relations of the bodies of the solar system, which have been shown by -astronomers to lend such support to the nebular hypothesis, have been -thought, at the same time, to favour the view that our earth is still -in a condition of uncompleted solidification. - -But it is quite admissible to accept the nebular hypothesis and the -view that our globe attained its present form while still in a state -of fluidity, and at the same time to maintain that our earth has long -since reached its condition of complete solidification. And there are -not a few facts which appear to lend support to such a conclusion. - -[Sidenote: SUPPOSED PROOFS OF LIQUID NUCLEUS.] - -If the rapid rate of increase in temperature which has been -demonstrated to occur at so many parts of the earth's surface be -maintained to the centre, then, as argued by David Forbes and Dana, it -is difficult to conceive of our earth as being in any other condition -than that of a liquid mass covered by a comparatively thin crust. The -objection to this view, both upon geological and astronomical grounds, -we have pointed out in the previous chapter. - -Before accepting as a demonstrated conclusion this notion of a constant -increase of temperature from the surface to the centre of our globe, it -may be well to re-examine the facts which are relied upon as proving it. - -That there is a general increase of temperature so far as we are able -to go downwards in the earth's crust, there can, as we have seen, -be no doubt whatever. Yet it may be well to bear in mind how very -limited is the range of our observation on the subject. The deepest -mines extend to little more than half-a-mile from the surface, and the -deepest borings to little more than three-quarters of a mile, while the -distance from the earth's surface to its centre is nearly 4,000 miles. -We may well pause before we extend conclusions, derived from such very -limited observations, to such enormous depths. - -But when we examine critically these observations themselves, we -shall find equal grounds for caution in generalising from them. There -is the greatest and most startling divergence in the results of the -observations which have been made at different points at the earth's -surface. Even when every allowance is made for errors of observation, -these discrepancies still remain. In some places the increase of -temperature as we go downwards is so rapid that it amounts to 1° -Fahrenheit for every 20 feet in depth, while in other cases, in order -to obtain the same increase in temperature of 1° Fahrenheit, we have to -descend as much as 100 feet. - -Now if, as is so often assumed, this increase of temperature as we -go downwards be due to our approach to incandescent masses forming -the interior portions of the globe, it is difficult to understand -why greater uniformity is not exhibited in the rate of increase in -different areas. No difference in the conducting powers of the various -rock-materials is sufficient to account for the fact that in some -places the rate of increase in temperature in going downwards is no -less than five times as great as it is in others. - -[Sidenote: VARIATIONS IN UNDERGROUND TEMPERATURES.] - -Again, there are some remarkable facts concerning the variation in -the rate of increase in temperature with depth which seem equally -irreconcilable with the theory that the heat in question is directly -derived from a great, central, incandescent mass. M. Walferdin, by a -series of careful observations in two shafts at Creuzot, proved that -down to the depth of 1,800 feet the increase of temperature amounted to -1° Fahrenheit for every 55 feet of descent, but below the depth named, -the rate of increase was as much as 1° Fahrenheit for every 44 feet. On -the other hand, in the great boring of Grenelle at Paris, the increase -in temperature down to the depth of 740 feet amounted to 1° Fahrenheit -for every 50 feet of descent, but from 740 feet down to 1,600 feet, -the rate of increase diminished to 1° for 75 feet of descent. The -same remarkable fact was strikingly shown in the case of the deepest -boring in the world--that of Sperenberg, near Berlin, which attained -the great depth of 4,052 feet. In this case, the rate of increase in -temperature for the first 1,900 feet, was 1° Fahrenheit for every 55 -feet of descent, and for the next 2,000, it diminished to 1° Fahrenheit -for every 62 feet of descent. In the deep well of Buda-Pesth there was -actually found a decline in temperature below the depth of 3,000 feet. - -Perhaps the most interesting fact in connection with this question -which has been discovered of late years, is that in districts which -have recently been the seat of volcanic agencies, the rate of increase -in temperature, as we go downwards in the earth's crust, is abnormally -high. Thus at Monte Massi in Tuscany, the temperature was found to -increase at the rate of 1° Fahrenheit for every 24 feet of descent. -In Hungary several deep wells and borings have been made, which prove -that a very rapid increase of temperature occurs. The deep boring at -Buda-Pesth penetrates to a depth of 3,160 feet, and a temperature of -178° Fahrenheit has been observed near the bottom. The rate of increase -of temperature in this boring was about 1° for every 23 feet of -descent. In the mines opened in the great Comstock lode, in the western -territories of the United States, an abnormally high temperature has -been met with amounting in some cases to 157° Fahrenheit. Although -this is the richest mineral-vein in the world, having yielded since -1859, when it was first discovered, 60,000,000_l._ worth of gold and -silver, this rapid increase in temperature in going downwards threatens -in the end to entirely baffle the enterprise of the miner. The rate -of increase in temperature in the case of the Comstock mines has been -estimated at 1° Fahrenheit for every 46 feet of descent, between 1,000 -and 2,000 feet from the surface, but as much as 1° Fahrenheit for every -25 feet, at depths below 2,000 feet. - -The facts which we have stated, with others of a similar kind, have -led geologists to look with grave feelings of doubt upon the old -hypothesis which regarded the increase of temperature found in making -excavations into the earth's crust as a proof that we are approaching a -great incandescent nucleus. They have thus been led to enquire whether -there are any conceivable sources of high temperatures at moderate -depths--temperatures which would be quite competent to produce locally -all the phenomena of volcanic action. - -There are not wanting other facts which seem to point to the same -conclusion: namely, that volcanic action is not due to the existence -of a universal reservoir of incandescent material occupying the central -portion of our globe, but to the local development of high temperatures -at moderate depths from the surface. - -[Sidenote: DEPTHS AT WHICH EARTHQUAKES ORIGINATE.] - -The close connection between the phenomena of volcanoes and earthquakes -cannot be doubted. It is true that some of those vibrations or tremors -of the earth's crust, to which we apply the name of earthquakes, -occur in areas which are not now the seat of volcanic action; and it -is equally true that the stratified rock-masses of our globe, far -away from any volcanic centres, exhibit proofs of violent movement -and fracture, in the production of which, concussions giving rise -to earthquake vibrations, could scarcely fail to have occurred. But -it is none the less certain that earthquakes as a rule take place -in those areas which are the seats of volcanic action, and that -great earthquake-shocks precede and accompany volcanic outbursts. -Sometimes, too, it has been noticed that the manifestation of -activity at a volcanic centre is marked by the sudden decline of the -earthquake-tremors of the district around, as though a safety-valve had -been opened at that part of the earth's surface. - -Mr. Mallet has shown that by the careful study of the effects -produced at the surface by earthquake-vibrations, we may determine -with considerable accuracy the point at which the shock or concussion -occurred which gave rise to the vibration. Now it is a most remarkable -fact that such calculations have led to the conclusion that, so far -as is at present known, earthquake shocks never originate at greater -depths than thirty miles from the surface, and that in some cases -the focus from which the waves of elastic compression producing an -earthquake proceed is only at the depth of seven or eight miles. -As we have already seen, there can be no doubt that in the great -majority of instances the forces originating earthquake-vibrations and -volcanic outbursts are the same, and independent lines of reasoning -have conducted us to the conclusion that these forces operate at very -moderate distances from the earth's surface. - -Under these circumstances, geologists have been led to enquire -whether there are any means by which we can conceive of such an -amount of heat, as would be competent to produce volcanic outbursts, -being locally developed at certain points within the earth's crust. -Recent discoveries in physical science which have shown the close -relation to one another of different kinds of force, and their mutual -convertibility, have at least suggested the possibility of the -existence of causes by which such high temperatures within certain -portions of the earth's crust may be originated. - -[Sidenote: DAVY'S CHEMICAL THEORY.] - -When, at the commencement of the present century, Sir Humphry Davy -discovered the remarkable metals of the alkalies and alkaline earths, -and at the same time demonstrated the striking phenomena which are -exhibited if these metals be permitted to unite with oxygen, he at -once perceived that if such metals existed in an uncombined condition -within the earth's crust, the access of water and air to the mass might -give rise to the development of such an amount of heat, as would be -competent to produce volcanic phenomena at the surface. It is true that -at a later date Davy recognised the chemical theory of volcanoes as -being beset with considerable difficulties, and was disposed to abandon -it altogether. It was argued, with considerable show of reason, that if -the heat at volcanic centres were produced by the access of water to -metallic substances, great quantities of hydrogen would necessarily be -evolved, and this gas ought to be found in prodigious quantities among -the emanations of volcanoes. The fact that such enormous quantities of -hydrogen gas are not emitted from volcanic vents has been held by many -authors to be fatal to the chemical theory of volcanoes. - -But the later researches of Graham and others have made known facts -which go far towards supplying an answer to the objections raised -against the chemical theory of volcanoes. Various solids and liquids -have been shown to possess the power of absorbing many times their -volume of certain gases. Among the gases thus absorbed in large -quantities by solids and liquids, hydrogen is very conspicuous. In -some cases gases are absorbed by metals or other solids in a state of -fusion, and yielded up again by them as they cool. - -It is a very remarkable circumstance that some meteorites are found -to have absorbed large quantities of hydrogen gas, and this is given -off when they are heated in vacuo. Thus it has been demonstrated that -certain meteorites have contained as much as forty seven times their -own volume of hydrogen gas. - -We have already pointed out that there are reasons for believing the -internal portions of our globe to be composed of materials similar to -those found in meteorites. If such be the case, the access of water -to these metallic substances may result in the formation of oxides, -attended with a great local development of heat, the hydrogen which -is liberated being at once absorbed by the surrounding metallic -substances. That this oxidation of the metallic substances in the -interior of our globe by the access of water and air from the surface -is continually going on, can scarcely be doubted. We may even look -forward to a far-distant period when the whole of the liquid and -gaseous envelopes of the globe shall have been absorbed into its -substance, and our earth thereby reduced to the condition in which we -now find the moon to be. - -There is a second method by which high temperatures might be locally -developed within the earth's crust, which has been suggested by Vose, -Mallet, and other authors. - -We have good grounds for believing that the temperature of our globe -is continually diminishing by its radiation of heat into space. This -cooling of our globe is attended by contraction, which results in -movements of portions of its crust. It may at first sight appear that -such movements would be so small and insignificant as to be quite -unworthy of notice. But if we take into account the vast size of our -earth it will be seen that the movements of such enormous masses may be -attended with the most wonderful results. - -It has been shown that if a part of the earth's crust fifty miles in -thickness were to have its temperature raised 200° Fahrenheit, its -surface would be raised to the extent of 1,000 or 1,500 feet Le Conte -has pointed out that if we conceive the conduction of heat to take -place at slightly different rates along different radii of our globe, -we should at once be able to account for the existing inequalities of -the earth's surface, and for all those continental movements which can -be shown to have taken place in past geological periods. - -[Sidenote: DYNAMICAL THEORIES.] - -But if we admit, as we have good grounds for doing, that the loss of -heat from the external portions of our globe goes on more rapidly than -in the case of the central masses, we have thereby introduced another -powerful agent for the production of high temperatures within the -earth's crust. The external shell of the globe will tend to contract -upon the central mass, and in so doing a series of tangential strains -will result which will be capable of folding and crumpling the rocks -along any lines of weakness. That such crushing and crumpling has -during all geological periods taken place along lines of weakness -in the earth's crust, is proved, as we have seen, by the phenomena -presented by mountain-ranges. Now these crushings, crumplings, and -other violent movements of great rock-masses must result in the -development of a vast amount of heat, just as the forcing down of a -break upon a moving wheel produces heat. This conclusion is strikingly -confirmed by the well-known geological fact that nearly all rocks which -have undergone great movement and contortion are found to present -evidence of having been subjected to such chemical and crystalline -actions, as would result from the development of a high temperature -within their mass. - -[Sidenote: RECAPITULATION OF SEVERAL THEORIES.] - -Let us sum up briefly the various methods which have been suggested to -account for the high temperatures within certain parts of the earth's -crust by which volcanic phenomena are produced. - -Our globe may be conceived of as an incandescent liquid mass surrounded -by a cooler, solid shell. If we regard this liquid interior mass as -supplying directly the various volcanic vents of the earth, it must be -conceded that the outer shell is of comparatively slight thickness. -But astronomers are almost universally agreed that such a thin outer -shell and inner liquid mass are quite incompatible with that rigidity -which our planet exhibits under the attractions of its neighbours. -Geologists are almost equally unanimous in regarding this hypothesis -of a liquid nucleus and thin, solid shell as contradicted by the -stability of the conditions which have been maintained during such -long past periods, and which exist at the present day. The extent and -character of volcanic action do not indicate a condition of general -instability in our earth, but one of stability subject to small and -local interferences The grandest volcanic disturbances appear small and -insignificant, if we take into account the vast dimensions of the globe -upon which they are displayed. - -If, on the other hand, we consider the outer solid shell to be of -great thickness, we are met by the difficulty of accounting for the -upheaval of liquid matter through such vast thicknesses of a solid -shell. The differences in character of lavas extruded from closely -adjoining volcanic districts seem equally difficult of explanation on -any theory of a central, fluid nucleus and a solid, outer shell. Nor is -the distribution of heat within the earth's crust so uniform as might -be anticipated, if the source of that heat be a great central mass of -highly heated materials. - -Under these circumstances, geologists and physicists have enquired -whether any other conditions can be imagined as existing in the earth's -interior, which would better account for the observed phenomena than -does the hypothesis of a liquid nucleus and a solid outer shell. Two -such alternative hypotheses have been suggested. - -Mr. Hopkins, adopting the theory that the earth has solidified both at -the centre and its outer surface, endeavoured to explain the occurrence -of volcanoes and earthquakes by supposing that cavities of liquid -material have been left between the solid nucleus and the solid shell, -and these cavities full of liquid material constitute the sources from -which the existing volcanoes of the globe draw their supplies. But -this hypothesis is found to be beset with many difficulties when we -attempt to apply it to the explanation of the phenomena of volcanic -action. It entirely fails, among other things, to account for the -remarkable fact that during past geological periods the scene of -volcanic action has been continually shifting over the surface of the -earth, so that there is probably no considerable area of our globe -which has not at one time or other been invaded by the volcanic forces. - -By some other theorists, who have felt the full force of this last -objection, an attempt has been made to explain the phenomena of -volcanoes by supposing that the globe is solid from its surface to its -centre, but that the internal portions of the globe are at such a high -temperature that they are only retained in a solid condition by the -enormous pressure to which they are subjected. The central masses of -the globe are thus regarded as being in an _actually_ solid, but in a -_potentially_ liquid condition, and any local relief of pressure is at -once followed by the conversion of solid to liquefied materials, in the -district where the relief takes place, resulting in the manifestation -of volcanic phenomena at the spot. It may be granted that this -hypothesis better accords with the known facts of Vulcanology than any -of those which we have previously described, but it is impossible to -shut our eyes to the fact that not a few serious difficulties still -remain. Thus it is based upon the assumption that the law of the -elevation of the point of fusion by pressure is true at temperatures -and pressures almost infinitely above those at which we are able to -conduct observations; but neither experiment nor analogy warrant this -conclusion, for the former shows that the elevation of the point of -fusion by pressure goes on in a continually diminishing ratio, and the -latter famishes us with the example of volatile liquids which, above -their critical points, obstinately remain in a gaseous condition under -the highest pressures. Nor is it easy upon this hypothesis to account -for the very irregular distribution of temperatures within the earth's -crust, as demonstrated by observations in mines, wells, and borings. -The hypothesis further requires the assumption that, at such very -moderate depths as are required for the reservoirs of volcanoes, the -effects of pressure and temperature on the condition of rock-materials -are so nicely balanced that the smallest changes at the surface lead to -a disturbance of the equilibrium. - -[Sidenote: DIFFICULTIES NOT YET EXPLAINED.] - -It is the weight of these several objections that has led geologists -in recent years to regard with greater favour those hypotheses which -seek to account for the production of high temperatures within parts of -the earth's crust, without having recourse to a supposed incandescent -nucleus. If it can be shown that there are any chemical or mechanical -forces at work within the crust of the globe which are capable of -producing local elevations of temperature, then we may conceive of a -condition of things existing in the earth's interior which is free -from the objections raised by the astronomer on the score of the -earth's proved rigidity, and by the geologist on the ground of its -general stability, and which at the same time seems to harmonise better -with the observed facts of the distribution of temperature within the -earth's crust. How far the existence of such chemical and mechanical -agencies capable of producing high temperatures within the crust of the -globe have been substantiated, we have already endeavoured to point out. - -It must be admitted, then, that the questions of the nature of the -earth's interior and the cause of the high temperatures which produce -volcanic phenomena, are still open ones. We have not yet got beyond the -stage of endeavouring to account for the facts observed by means of -tentative hypotheses. Some of these, as we have seen, agree with the -facts, so far as they are at present known, much better than others; -but the decision between them or the rejection of the whole of them in -favour of some new hypothesis, must depend on the results of future -observation and enquiry. - -It may be well, before leaving this subject, to remark that they are -all equally reconcilable with the nebular theory of Kant and Laplace. -Granting that the matter composing our globe has passed successively -through the gaseous and liquid conditions, it is open to us to imagine -the earth as now composed of a liquid nucleus with either a thick or -a thin solid shell; of a solid nucleus and a solid shell with more or -less liquid matter between them; or, lastly, to conceive of it as -having become perfectly solid from the centre to the surface. - -[Sidenote: CAUSE OF THE PRESENCE OF WATER IN LAVAS.] - -But it is not upon the existence of a high temperature within certain -parts of the earth's crust that the production of volcanic activity -alone depends. The presence of water and other liquid and gaseous -substances in a state of the most intimate admixture with the fused -rock-masses, is, as we have seen, the main cause of the violent -displays of energy exhibited at volcanic centres. And We shall now -proceed to notice the hypotheses which have been suggested to account -for the presence of these liquid and gaseous bodies in the midst of the -masses of incandescent materials poured out from volcanic vents. - -There is an explanation of this presence of water and various gases in -the masses of molten rock-materials within the earth's crust which at -once suggests itself, and which was formerly very generally accepted. -Volcanoes, as we have seen, are usually situated near coast-lines, and -if we imagine fissures to be produced by which sea-water finds access -to masses of incandescent rock-materials, then we can regard volcanic -outbursts as resulting from this meeting of water with rock-masses in a -highly healed condition. This supposition has been thought to receive -much support from the fact that many of the gases evolved from volcanic -vents are such as would be produced by the decomposition of substances -present in sea-water. - -But it frequently happens that an explanation which at first sight -appears to be very simple and obvious, turns out on more critical -examination to be quite the reverse, and this is the case with the -supposed origination of volcanic outbursts by the access of sea-water -to incandescent rock-material by means of earth-fissures. It is -difficult to understand how, by such means, that wonderfully intimate -union between the liquefied rock and the water, evolved in such -quantities during volcanic outbursts, could be brought about; and -moreover, we can scarcely regard the production of fissures in the -earth's crust as being at the same time both the cause and the effect -of this influx of water to the deep-seated rock-masses at a high -temperature. - -[Sidenote: ABSORPTION OF GASES BY LIQUIDS AND SOLIDS.] - -During recent years the attention of both geologists and physicists -has been directed to a remarkable property exhibited by many liquids -and solids, as supplying a possible explanation of the phenomena of -volcanic action. The property to which we refer is that whereby some -liquid and solid substances are able to absorb many times their volume -of certain gases--which gases under different conditions may be given -off again from the liquids or solids. This power of absorption is a -very remarkable one; it is not attended with chemical combination, but -the amount of condensation which gases must undergo within the solid or -liquid substances is sometimes enormous. Water may be made to absorb -more than 1,000 times its volume of ammonia, and more than 500 times -its volume of hydrochloric acid. Alcohol may absorb more than 300 -times its volume of sulphurous acid. Charcoal may absorb 100 times its -volume of ammonia, 85 times its volume of hydrochloric acid, 65 times -its volume of sulphuretted hydrogen, 55 times its volume of sulphurous -acid, and 35 times its volume of carbonic acid. Platinum-black absorbs -many times its volume of oxygen and other gases. - -This power of absorption of gases varies in different solids and -liquids according to the conditions to which they are subjected. Dr. -Henry showed it to be a general law in liquids that, as the pressure is -augmented, the weight of the gas absorbed is proportionately increased. - -Sometimes this absorption of gases takes place only at high -temperatures. Thus silver in a state of fusion is able to absorb 22 -times its volume of oxygen gas. When the metal is allowed to cool this -gas is given off, and if the cooling takes place suddenly a crust is -formed on the surface, and the phenomenon known as the 'spitting of -silver' is exhibited. Sometimes during this operation miniature cones -and lava-streams are formed on the surface of the cooling mass, which -present a striking resemblance to those formed on a grand scale upon -the surface of the globe. Similar phenomena are exhibited by several -other metals and by the oxide of lead. - -The researches of Troost and others have shown that molten iron and -steel possess the property of absorbing considerable quantities of -oxygen, hydrogen, carbonic acid, and carbonic oxide, and that these -gases are given off in the operation known as 'seething,' when either -the pressure or the temperature is diminished. - -Hochstetter has shown that in the process of extracting sulphur from -the residues obtained during the manufacture of soda, some very -interesting phenomena are manifested. The molten sulphur is exposed -to a temperature of 262° Fahrenheit, and a pressure of two or three -atmospheres, in the presence of steam; under these circumstances it -is found that the sulphur absorbs a considerable quantity of water, -which is given off again with great violence from the mass as it -undergoes solidification. The hardened crust which forms on the surface -of the molten sulphur is agitated and fissured, miniature cones and -lava-streams being formed upon it, which have a striking resemblance to -the grander phenomena of the same kind exhibited upon the crust of the -globe. - -The observations which we have described prove conclusively that many -liquids and solids in a molten condition have the power of absorbing -many times their volume of certain gases, and that this action is aided -by heat and pressure. - -That the molten materials which issue from volcanic vents have -absorbed enormous quantities of steam and other gases, we have the -most undisputable evidence. The volume of such gases given off -during volcanic outbursts, and while the lava-streams are flowing -and consolidating, is enormous, and can only be accounted for by -supposing that the masses of fluid rock have absorbed many times their -volume of the gases. But we have another not less convincing proof -of the same fact in the circumstance that volcanic materials which -have consolidated under great pressure--such as granites, gabbros, -porphyries, &c.--exhibit in their crystals innumerable cavities -containing similar gases in a liquefied state. - -It is to the violent escape of these gases from the molten rock-masses, -as the pressure upon them is relieved, that nearly all the active -phenomena of volcanoes must be referred; and it was the recognition of -this bet by Spallanzani, while he was watching the phenomena displayed -in the crater of Stromboli, which laid the foundations of the science -of Vulcanology. - -[Sidenote: SOURCE OF THE ABSORBED GASES.] - -But here another question presents itself to the investigator of the -phenomena of volcanoes: it is this. At what period did the molten -rock-masses issuing from vents absorb those gaseous materials which -are given off so violently from their midst during eruptions? Two -different answers to this question have been suggested. It may be that -the original materials of which our globe was composed consisted of -metallic substances in a state of fusion which had absorbed many gases, -and that, in the fluid masses below the solid crust, vast quantities of -vapour and gas are stored up, which are being gradually added to the -atmosphere during volcanic outbursts. The fact that meteorites, which, -as we have seen, in all probability closely resemble the materials -forming the earth's interior, sometimes yield many times their volume -of hydrogen and other gases, may be thought to lend some support to -this idea. If it be the correct one, we must regard our globe as -gradually parting with its pent-up stores of energy, in those absorbed -gases and vapours held in bondage by the solid and fluid materials of -its interior. - -But there is another hypothesis which is, to say the least, equally -probable. Water containing various gases in solution is continually -finding its way downwards by infiltration into the earth's crust. -Much of this water, after passing through pervious beds, reaches -some impervious stratum and is returned to the surface in the form -of springs. But that some of this percolating water penetrates to -enormous depths is shown by the fact that the deepest mines and borings -encounter vast underground supplies of water. When we remember that -nearly three-fourths of the earth's surface is covered by the waters -of the ocean, and that the average depth of these oceanic waters is -more than 10,000 feet, we may easily understand how great a portion of -the earth's crust must be penetrated by infiltrating waters which can -find no outlet in springs. The penetration of the waters of the ocean -into the earth's crust will be aided, too, by the enormous pressure -amounting to not less than several tons to the square-inch upon the -greater part of the ocean-floor. It might be thought that this downward -penetration of water would be counteracted by the upward current of -steam that would be produced as these subterranean waters reach the -hotter portions of the earth's crust. But the experiments of Daubrée -have conclusively shown that the penetration of water through rocks -takes place in opposition to the powerful pressure of steam in the -contrary direction. Hence, we may assume that certain quantities of -water, containing various gases and solids in solution, are continually -finding their way by capillary infiltration from the surface to -the deeply seated portions of the earth's crust, there to undergo -absorption by the incandescent rock-masses and to produce oxidation of -some of their materials. - -[Sidenote: POSITION OF THE ISOGEOTHERMS.] - -The deep-sea soundings of the 'Challenger' have shown that the floor of -the ocean is constantly maintained at a temperature but little above -that of the freezing point of water. This low temperature is probably -produced by the absorption of heat from the earth's crust by the waters -of the ocean, which distribute it by means of convection currents on -the grandest scale. Hence, the isogeotherms, or lines indicating the -depths at which the same mean temperature is found within the earth's -crust, are probably depressed beneath the great ocean-floors, and rise -towards the land-masses. It is to this circumstance, combined with -that of the enormous pressure of water on the ocean-beds, that we must -probably ascribe the general absence of volcanoes in the deep seas and -their distribution near coast-lines. - -We have thus briefly reviewed the chief hypotheses which have been -suggested in order to account for the two great factors in all volcanic -phenomena--namely, the presence of highly heated rock-masses within -the earth's crust, and the existence of various vapours and gases in a -state of most intimate mechanical, but not chemical, union with these -incandescent materials. It must be admitted that we do not at present -appear to have the means for framing a complete and consistent theory -of volcanic action, but we may hopefully look forward to the time when -further observation and experiment shall have removed many of the -existing difficulties which beset the question, and when by the light -of such future researches untenable hypotheses shall be eliminated and -the just ones improved and established. - -But if we are constrained to admit that a study of the observed -phenomena and established laws of volcanic action have not as yet -enabled us to frame any complete and satisfactory theory on the -subject, we cannot lose sight of the fact that all modern speculation -upon this question appears to be tending in one definite direction. It -is every day becoming more and more clear that our earth is bound by -ties of the closest resemblance to the other members of that family of -worlds to which it belongs, and that the materials entering into their -constitution, and the forces operating in all are the same. - -We have had occasion in a previous chapter to point out that there -are the strongest grounds for believing the interior of our globe to -consist of similar materials to those found in the small planetary -bodies known as meteorites. That the comets are merely aggregations -of such meteorites, and that the planets differ from them only in -their greater dimensions, may be regarded as among the demonstrated -conclusions of the astronomer. The materials found most abundantly in -meteorites and in the interior of our globe are precisely the same as -those which are proved to exist in an incandescent state in our sun. -Hence we are led to conclude that the whole of the bodies of the solar -system are composed of the same chemical elements. - -[Sidenote: ERUPTIVE ACTION IN THE SUN.] - -That the forces operating in each of these distant bodies present -striking points of analogy is equally clear. The sun is of far greater -dimensions than our earth, and is still in great part, if not entirely, -in a gaseous condition. The great movements in the outer envelopes of -the sun exhibited in the 'sun-spots' and 'solar prominences,' recall -to the mind the phenomena of volcanic activity upon our globe. But -the vast energy still existing in the intensely heated mass of the -sun, and the wonderful mobility of its gaseous materials, give rise to -appearances beside which all terrestrial outbursts seem to sink into -utter insignificance. Vast cavities of such dimensions that many globes -of the size of our earth might be swallowed up in them are formed -in the solar envelopes in the course of a few days or hours. Within -these cavities or sun-spots incandescent vapours are observed, rushing -upwards and downwards with almost inconceivable velocity. - -The drawings made by Secchi, and reproduced in figs. 89 and 90, will -give some idea of the appearances presented by these great holes in the -solar envelopes. - -[Illustration: Fig. 89.--A group of Sun-spots. (After Secchi.)] - -In fig. 89 a group of sun-spots is represented and, in their circular -outlines and tendency to a linear arrangement, they can scarcely fail -to remind anyone familiar with volcanic phenomena of terrestrial -craters, though their dimensions are so much greater. - -In fig. 90 the sun-spot represented shows the presence of large -floating masses of incandescent materials rushing upwards and downwards -within the yawning gulf. - -[Sidenote: PHENOMENA OF SUN-SPOTS.] - -[Illustration: Fig. 90.--A Sun-spot, showing the great masses of -incandescent vapour rising or falling within it. (After Secchi.)] - -[Illustration: Fig. 91.--The edge of a Sun-spot, showing a portion of -the prominent masses of incandescent gas (A), which detached itself at -E and floated into the midst of the cavity.] - -From fig. 91, taken from a drawing by Mr. Norman Lockyer, we may -understand the movements of these great protuberances of incandescent -gas which are seen on the sides of the sun-spots. - -The so-called solar prominences present even more striking resemblances -to the volcanic outbursts of our globe. - -Two drawings made by Mr. Norman Lockyer will serve to give some idea -of the vast dimensions of these solar prominences, and of the rapid -changes which take place in their form. - -[Illustration: Fig. 92.--Drawing of a Solar prominence, made by Mr. -Norman Lockyer on March 14, 1869, at 11 H. 5 M. A.M.] - -The masses of incandescent gas were estimated as being no less than -27,000 feet in height, yet in ten minutes they had totally changed -their form and appearance, as shown in fig. 93. - -Even still more striking are the changes recorded by Professor Young, -of New-Haven, in a solar prominence, which he observed on September 7, -1871. - -[Illustration: Fig. 93.--The same object, as seen at 11 H. 15 M. on the -same day.] - -[Sidenote: SOLAR PROMINENCES.] - -That astronomer described a mass of incandescent gas rising from -the surface of the sun to the height of 54,000 miles. In less than -twenty-five minutes he saw the whole mass torn to shreds and blown -upwards, some of the fragments being in ten minutes hurled to the -height of 200,000 miles above the sun's surface. The masses of -incandescent gas thus hurled upwards were of enormous dimensions, the -smallest being estimated as having a greater area than the whole of the -British Islands, and the force with which they were urged upwards was -so great that they acquired a velocity of 166 miles per second. The -accompanying woodcut shows the successive appearances presented by this -grand eruptive outburst on the surface of the sun. - -[Illustration: Fig. 94.--Drawings of a Solar prominence at four -different periods on Sept. 7, 1871. (After Young.)] - -[Sidenote: EXTINCT VOLCANOES OF THE MOON.] - -The moon, which is of far smaller size than our earth, exhibits on -its surface sufficiently striking evidences of the action of volcanic -forces. Indeed the dimensions of the craters and fissures which cover -the whole visible lunar surface are such that we cannot but infer -volcanic activity to have been far more violent on the moon than it -is at the present day upon the earth. This greater violence of the -volcanic forces on the moon is perhaps accounted for by the fact that -the force of gravity on the surface of the moon is only one-sixth of -that at the surface of the earth; and thus the eruptive energy will -have a much less smaller resistance to overcome in bursting asunder the -solid crust and accumulated heaps of ejected materials on its surface. -But the volcanic action on the moon appears now to have wholly ceased, -and the absence of both water and atmosphere in our satellite suggests -that this extinction of volcanic energy may have been caused by the -complete absorption of its gaseous envelope. The appearance presented -by a portion of the moon's surface is shown in fig. 95. - -The sun and the moon appear to exhibit two widely separated extremes -in the condition assumed during the cooling down from a state -of incandescence of great globes of vaporised materials. The -several planets, our own among the number, probably exhibit various -intermediate stages of consolidation. - -[Illustration: Fig. 95.--A group of Lunar craters (Maurolycus, -Barocius, etc.), the largest being more than 60 miles in diameter.] - -[Sidenote: ERUPTIVE ACTION IN THE SUN, EARTH AND MOON.] - -Our earth is, as we have seen, closely allied to the other bodies of -the solar system in its movements, its relations, and its composition; -and a true theory of terrestrial vulcanicity, when it is discovered, -may be expected not only to afford an explanation of the phenomena -displayed on our own globe, but to account for those displays of -internal energy which have been manifested in other members of the same -great family of worlds. - - - - -INDEX. - - -[The subjects illustrated in the engravings are indicated by _italics_, -the names of authors are in Capitals.] - - ABICH, cited, 122 - -- researches of, 4 - Absorption of gases by liquids and solids, 354, 355 - Acid lavas, 48 - Æolian Islands. _See_ Lipari Islands - Æolus, origin of myth, 35 - Africa, volcanoes of, 227 - -- South, diamonds of, 147 - Agates, formation of, 150 - Allport, Mr., cited, 259 - Alps, formation of, 292 - Altered lavas, names given to, 261 - America, volcanoes of, 227 - Amygdaloids, formation of, 140, 141 - Andesites, 50, 59 - Andesite-volcanoes, 126 - Andrews, Professor, cited, 321 - Anne Boleyn and Etna, 3 - Armstrong, Sir W., hydro-electric machine, 29 - Arthur's Seat, 275 - Artificial stone, 55 - Asia, volcanoes of, 227 - Asiderites, 316 - Asmanite, 314 - Astroni, crater-ring of, 170 - Atlantic, volcanoes in, 223 - Auvergne, _breached cones of_, 123, fig. 40 - -- _denuded cones in_, 124, fig. 42 - -- incrusting springs of, 184 - -- puys of, 152, 212 - -- volcanic cones in, 79 - - BALL-AND-SOCKET structure in basaltic columns, 107 - Barrancos, formation of, 209 - Basalt, controversy concerning origin of, 249 - Basalts, 49, 50, 59 - Basaltic columns of Bohemia, 107 - -- -- of Central Germany, 107 - -- -- of Monte Albano, 107 - -- -- _from the Giant's Causeway_, 107, fig. 29 - Basic lavas, 48 - Bath, hot spring of, 219 - Ben Nevis, 274 - Bohemia, volcanoes of, 126 - -- lavas of, 103 - Boiling. _See_ Ebullition - Bonney, Professor, cited, 69, 109, 259 - Boracic acid at volcanic vents, 216 - _Bourbon, volcano of_, 176, figs. 74, 75 - _Bracciano, crater-lake of_, 178, fig. 77 - _Breached cones_, 123, fig. 40 - Babbles of steam, escape from lava, 21 - _Bubbles, spontaneous movement of, in liquid cavities_, 62, fig. 8 - -- -- cause of, 65 - Buch, Von, researches of, 4 - Buda-Pesth, deep well of, 335, 341 - Büdos Hegy, Transylvania, 215 - Bunsen, cited, 201 - Burning, does not take place at volcanoes, 2 - - CADER IDRIS, 274 - 'Calderas,' formation of, 180 - Caldera of Palma, 209 - Cambro-Silurian volcanoes of British Islands, 274 - _Campi-Phlegræi, map of_, fig. 11 - -- -- volcanoes of, 79 - -- -- tuff-cones of, 118 - -- -- fissures in, 197 - Carbonic acid in cavities of crystals, 63 - Carboniferous volcanoes of British Islands, 275 - Carlsbad, Strudel of, 218 - -- Strudelstein of, 184 - Caspian Sea, mud-volcanoes of, 182 - Catacecaumene, volcano cones in, 79 - Cause of proximity of volcanoes to sea, 239 - Central Asia, volcanoes of, 236 - -- America, mud-volcanoes, 182 - -- Pacific, volcanoes of, 236 - 'Challenger,' H.M.S., voyage of, 73 - -- -- soundings of, 359 - Chance, Messrs., of Birmingham, 55 - Charnwood Forest, ancient volcanic rocks of, 259 - Chemical deposits at Vulcano, 44 - -- -- on surfaces of lavas, 110 - -- elements present in lavas, 46 - -- theory of volcanoes, 344, 346 - Chiaja di Luna, 108 - Chimborazo, size of, 44 - -- 151 - Chodi-Berg, Hungary, 161 - _Citlaltepetl, view of_, 169, fig. 69 - Coast-lines, proximity of volcanoes to, 228 - Cole, Mr. Grenville, 110 - Colours of lavas, 49 - Columns in lava, 105 - -- -- dimensions of, 105 - -- radiating in intrusive masses, 136 - Columnar structure in lavas, 104 - -- -- origin of, 105 - _Columnar lava-stream on the Ardèche_, 107, fig. 28 - Combustion, does not take place at volcanoes, 2 - Composite cones, 128, 161 - Comstock mines, temperature of, 342 - _Concentric jointing in lava_, 108, fig. 30 - _Cones composed of viscid lava_, 129, fig. 43 - -- miniature on lava-streams, 100, 101, figs. 25, 26 - -- natural sections of, 129 - -- shifting of axis in, 167 - Coolin Hills, Skye, 144 - Cotopaxi, volcanic dust of, 69 - -- _view of_, 168, fig. 68 - Craters, formation of, 82 - -- origin of, 167 - -- position of, 167 - -- fissuring of sides, 180 - Crater of Stromboli, aperture at bottom of, 15 - Crater-lakes, formation of, 171 - -- of Agnano, 171 - -- of Albano, 171 - -- of Avernus, 171 - -- _of Bagno_, 171, fig. 71 - -- of Bolsena, 171 - -- of Bracciano, 171 - -- of Frascati, 173, 175 - -- _of Gustavila_, 171, fig. 72 - -- of Laach, 171 - -- of Nemi, 171 - Crater-rings, formation of, 170 - _Crater-ring of Somma_, 177, fig. 76 - Crater-ring of Pianura, 174 - -- -- of Piano di Quarto, 174 - -- -- of Vallariccia, 174 - Creuzot, shafts at, 340 - 'Critical point' of liquids, 63 - Crust of globe, definition of, 308 - Crystals in lavas, 51 - -- -- formed of crystallites, 54-57 - -- -- formed in subterranean reservoirs, 60 - -- -- interruption in growth of, 60 - -- pressure under which formed, 65 - -- deposited on surface of lava, 110 - -- porphyritic, origin of, 256 - Crystalline minerals formed beneath volcanoes, 146, 147 - -- -- ejected from volcanoes, 147 - Crystallised minerals of volcanoes, 46 - Crystallites, aggregates of, 54, _Frontispiece_ - Crystallites in lavas, 53, _Frontispiece_ - Crypto-crystalline base, 57 - 'Cupolas,' 135 - Corral of Madeira, 209 - - DACITES, 198 - Dana, Professor, J. D., cited, 100, 159, 291, 301, 327, 338, 339 - Darwin, Mr., cited, 245, 246, 271, 289 - Daubeny, cited, 182 - Daubrée, M., cited, 147, 315, 320, 358 - Daubréelite, 314 - Davy, Sir Humphry, chemical theory of volcanoes, 344, 345 - Deccan of India, 103 - Density of the earth, 306 - _Denuded cones and craters_, 158, fig. 59 - Denudation, effects of, on volcanoes, 114 - Deposits about volcanic fissures, 42 - Detonations at Vesuvius, 26 - Devonian volcanoes of British Islands, 274 - Diorite, 59 - Dolomieu, cited, 4, 39 - Durocher, cited, 201 - Dykes, formation of, 116, 117, 209, 210 - -- structure of rock in, 211 - -- pseudo-, 119 - Dynamical theory of volcanoes, 347, 348 - - EARTH'S interior, nature of, 309 - -- -- physical condition of, 325 - -- -- hypothesis concerning, 328-330 - -- relation to other planets, 310, 311 - Earthquakes, depth of origin of, 343, 344 - -- connection with volcanoes, 343 - -- accompanying Vesuvian eruption of 1872, 27 - Ebullition, compared to volcanic eruptions, 19, 20 - Eifel, volcanic cones of, 45 - Ejected blocks, 45 - -- materials, height to which thrown, 72 - -- -- stratification of, 117-119 - Elements, pyroxenic and trachytic, theory of, 201 - Elevation-craters, theory of, 135, 200 - Erroneous opinions, sources of, in regard to volcanoes, 2 - Eruptions, feeble and violent compared, 31 - -- prediction of, not possible, 32 - -- intervals between, 33 - -- of varying intensity, 33 - -- and barometric pressure, 36 - -- effects of repetition of from same fissure, 80 - Eruptive action in sun and moon, 360-369 - Etna, ideas of ancients concerning, 3 - -- and Anne Boleyn, 3 - -- observatory on, 37 - -- size of, 44 - --, 151 - -- eruptions at summit and on flanks, 207 - _Etna, dyke and lava-stream in_, 133, fig. 54 - _Etna, views of_, 162, 163, figs. 62, 63 - Euganean Hills, 139 - -- -- volcanoes of, 201 - Europe, volcanoes of, 227 - Extra-terrestrial rocks, 316 - _Extra-terrestrial rocks, relation to ultra-basic rocks_, 322, fig. 88 - - FELSTONES, 263 - Ferric-chloride, mistaken for sulphur, 41 - _Fissure on flanks of Etna_, 194, fig. 84 - Fissure-eruptions, 188 - Fissures, volcanic cones on, 194 - -- systems of, 198 - Fingal's Cave, 106 - Flames, phenomena mistaken for, 2 - -- at volcanoes, feebly luminous, 17 - -- false appearance of, in volcanoes, 17 - Flames at volcanic vents, 41 - Flashing lighthouse, compared to Stromboli, 10 - Floods, accompanying volcanic outbursts, 30 - Forbes, Mr. David, cited, 337, 339 - Fossils, from beneath Vesuvius, 45 - -- supposed in basalt, 250 - Fouqué, M., cited, 110, 213 - Fumaroles, gases emitted from, 213 - Fusiyama, form of, 90, 166 - _Fusiyama_, 178, fig. 77 - - GABBRO, 59 - Gardiner's river, travertine terraces of, 185 - Gases emitted from volcanoes, 40 - -- -- volcanic vents, 212-216 - Geanticlinals, formation of, 297 - Gems, formation of, 147 - -- mode of occurrence, 148 - Geological continuity, doctrine of, 247 - Geosynclinals, formation of, 294 - Geysers, formation of, 217 - -- intermittent action of, 218 - -- of Colorado, 184, 217 - -- of Iceland, 184, 217 - Giant's Causeway, 108 - Gilbert, Mr. G. K., cited, 208 - Girgenti, mud-volcanoes of, 182 - Glass, formed by fusion of lavas, 52 - Glasses, composed of certain silicates, 58 - Glassy base, 57 - Goethe, cited, 112 - _Graham Isle_, 178, 179, fig. 78 - Graham, cited, 345 - Grand Sarcoui, Auvergne, 161 - Granite, 59 - Granite of Secondary and Tertiary ages, 254 - Granitic rocks, position beneath volcanoes, 145 - Great earth movements, nature of, 286 - Great volcanic bands of the globe, 232-234 - Grenelle, boring of, 341 - Greystones, 49 - Groundmass of lavas, 52 - Grotto del Cane, 215 - Guevo Upas, Java, 215 - Guiscardi, Professor, referred to, 45 - _Gustavila, crater-lake of_, 172, fig. 72 - - HAMILTON, Sir W., researches of, 4, 75, 84 - -- -- observations on Vesuvius, 80 - Hannay, Mr., referred to, 147 - Hartley, Mr. Noel, referred to, 65 - Hawaii, volcanoes of, 100, 125 - -- -- lava-masses of, 159 - -- -- volcanic eruptions at different levels, 327 - Hebrides, volcanoes of, 271 - Henry, Dr., cited, 355 - Henry Mountains, Southern Utah, 208 - Hephæstus, forge of, 3 - Hochstetter, cited, 135, 356 - Holosiderites, 315 - Hopkins, Mr., cited, 349 - Hot springs, numbers of, 219 - Humboldt, researches of, 4 - Hungary, lavas of, 96, 103 - -- volcanoes of, 126, 201 - -- deep wells of, 341 - _Hverfjall, Iceland_, 178, fig. 77 - Hydro-electric machine of Sir W. Armstrong, 29 - Hypothesis, value of, 331-333 - - ICE under lava of Vesuvius in 1872, and of Etna, 110 - Iceland, volcanic dust of, carried to Norway, 72 - Indian Ocean, volcanoes in, 229 - _Insel Ferdinandez_, 178, 179, fig. 78 - Intermediate lavas, 48 - Intervals between Eruptions, 33 - Ireland, north-east of, 103 - _Iron in Ovifak-basalts_, 319, fig. 87 - Iron, seething of, 356 - -- of Ovifak, terrestrial origin of, 320 - Ischia, eruption in 1301, 164 - -- _crater-lake of Bagno in_, 172, fig. 71 - -- _plan of_, 163, fig. 64 - -- _parasitic cones in_, 164, fig. 65 - Island of Bourbon, 93 - _Isle Julie_, 178, 179, fig. 78 - Isogeotherms, 359 - - JANSSEN, referred to, 42 - Joint-structures in lava, 104-110 - - _KAMMERBÜHL_, 112-114, fig. 33 - -- _section of_, 114, fig. 34 - -- _section in side of_, 118, fig. 36 - Kant, nebular hypothesis of, 352 - Kilauea, volcano of, 71, 138 - -- crater of, 181 - King, Mr. Clarence, cited, 301 - - LAACHER SEE, minerals ejected at, 149 - _Lac Paven, Auvergne_, 171, fig. 70 - 'Laccolites,' formation of, 208 - Lago di Bolsena, 173, 175 - Lago di Bracciano, dimensions of, 172, 173 - Lake Avernus, 215 - Lapilli, 70 - Laplace, nebular hypothesis of, 325, 352 - Lavas, action of acid gases on, 41 - -- resemblance to slags, 46 - -- chemical elements in, 46 - -- oxygen in, 47 - -- silicon in, 47 - -- proportion of silica and other oxides in, 47 - -- silicates in, 47 - -- acid, intermediate, basic, 48 - -- specific gravities of, 49 - -- colours of, 49 - -- microscopic study of, 50 - -- fusibility of, 51 - -- minerals in, 51 - -- artificially fused, 51 - -- crystals in, 51, 93 - -- ground mass of, 52 - -- crystalline forms of, 59 - -- of Bohemia, 103 - -- of Hungary, 96, 103 - -- of Kilauea, 95 - -- of Lipari, 96 - -- of Niedermendig, 103 - -- of Vesuvius, 104 - -- of Volvic, 95 - -- of Volcano, 95 - -- presence of water In, 102 - -- chemical deposits on, 110 - -- different fluidity of, 204 - -- augite and hornblende in, 267 - _Lava, cascade of_, 93, fig. 18 - Lava-cones, composed of liquid lava, 125 - -- -- of viscid lava, 126, 127 - -- characters of, of liquid lava, 159 - -- -- of viscid lava, 160 - _Lava-cones, outlines of_, 160, fig. 60 - Lava, in deep-seated reservoirs, 138 - -- consolidation of, at great depths, 139 - Lava-fountains, 94 - _Lava-sheets, intrusive_, 136, 137, fig. 56 - 'Lava' ornaments of Naples, 45 - 'Lava,' slow-cooling of, 110 - -- a bad conductor of heat, 110 - -- ice under, 110 - Lava-streams, nature of movements, 92 - -- difference in liquidity of, 92 - -- miniature cones on, 100, 101 - -- vast dimensions of, 102 - -- structure of, 103 - -- position of columns in, 106 - -- sinking of surface of, 111 - 'Lave di fango,' 30 - 'Lave di fuoco,' 30 - Lawrencite, 314 - Laws of volcanic action, 38 - Le Conte, cited, 347 - Leucite, absence from ancient lavas, 268 - Lightning, accompanying volcanic outbursts, 28 - Linear arrangement of volcanic vents, 191 - -- -- of volcanoes, 231 - Lipari Islands, 3, 39 - -- -- fissures in, 197 - -- -- pumice-cones in, 154 - -- -- order of appearance of lavas in, 200 - -- -- _breached pumice-cones in_, 124, fig. 41 - -- -- _map of_, 192, fig. 81 - -- -- _lavas of_, 96, figs. 20, 21 - Liquids in cavities of crystals, 63 - _Liquid cavities in lavas_, 60, fig. 7 - -- -- _spontaneous movement of bubbles in_, 62, fig. 8 - -- -- spontaneous movement of bubbles in, cause of, 65 - Lockyer, Mr. Norman, cited, 322, 363, 364 - _Lunar craters_, 368, fig. 95 - Lyell, Sir Charles, cited, 135, 167, 197 - - MACCULLOCH, cited, 207, 208 - _Madeira, cliff-section in_, 128, fig. 47 - Magmas, theory of, 201 - -- objections to, 202, 203 - Mallet, Mr., cited, 269, 343, 346 - _Mamelons of Bourbon_, 126, 127, figs. 45, 46 - Maskelyne, Professor, cited, 314 - Massa di Somma, destruction of, 26 - Mauna Loa, 138 - Metamorphism around volcanic vents, 145 - Meteorites, nature of, 312 - -- composition of, 313 - -- minerals of, 314 - -- classification of, 315 - Melaphyres, 262 - Miascite, 59 - Michel Lévy, M., 110 - Micro-crystalline base, 58 - Microliths. _See_ Crystallites - Microscopic study of lavas, 50 - Minerals in lavas, 51 - -- of Vesuvius, 46 - Mineral-veins, formation of, 149 - -- connection with volcanoes, 220 - -- nature of materials in, 321 - _Misenum, Cape of, section of tuff-cone of_, 121, fig. 38 - Modena, mud-volcanoes of, 182 - _Mont Dore, section at_, 130, fig. 48 - Monte Cerboli, Tuscany, 216 - Monte Massi, Tuscany, well at, 341 - Monte Nuovo, history of formation of, 76 - -- -- _description of_, 77, 78, fig. 10 - -- -- 152 - -- -- crater of, 168 - -- -- production of fissure at, 190 - Monte Rotondo, Tuscany, 216 - Moon, effect of internal forces on, 305 - Mountains, all volcanoes not, 2 - Mountain-chains, formation of, 291 - -- -- all of recent date, 292 - Mud-streams at volcanoes, 30 - Mud-volcanoes, formation of, 181, 182 - _Mull, dissected volcano of_, 142-4, figs. 57, 58 - Muscovite, absence of, from modern lavas, 268 - - NEBULAR hypothesis of Laplace, 325, 352 - -- -- of Kant, 352 - New Zealand, geysers of, 217 - -- -- volcanoes of, 135 - -- -- volcanic cones in, 79 - Niedermendig, lava of, 103 - Nordenskiöld, Professor, cited, 318 - - OBSERVATORY on Vesuvius, 24, 37 - -- on Etna, 37 - Obsidian, 59 - Oceans, depth of, in volcanic areas, 242 - Oceanic islands, volcanoes in, 228 - Oliver, Capt. S. P., 92 - Oldhamite, 314 - _Outlines of Vesuvius_, 87, fig. 17 - Ovifak, iron-masses of, 319 - Oxidation of materials of globe, 324 - Oxygen, proportion in lavas, 47 - - PACIFIC, volcanoes in, 229 - Palmieri, Professor, cited, 25, 37 - Papandayang, eruption of, 169 - Papin's digester, nature of action in, 22 - _Parasitic cones, formation of_, 161, 162, fig. 61 - Pele's Hair, 71 - Perlitic structure, 109 - Phillips, Mr. J. A., cited, 220 - Phonolites, 50, 59 - Phonolite-volcanoes, 126 - _Photograph of Vesuvius eruption_, 24, fig. 5 - 'Pine-tree, appendage of Vesuvius, 29 - Pitchstones, porphyritic, 60 - Plateaux formed of lava-sheets, 270 - Pliny, Elder, death of, 7 - Plombières, hot springs of, 147 - Plutonic rocks, 61 - Pompeii, nature of materials covering, 117 - Ponza Islands, 39 - _Ponza, sections in_, 131, 132, figs. 51, 52 - Porphyrites, 263 - Porphyritic pitchstones, 60 - Potentially liquid rock, 250 - Pre-Cambrian volcanoes of British Islands, 274 - Presence of water in lavas, 353 - Pressure under which crystals were formed, 65 - _Predazzo, ancient volcano of_, 165, fig. 67 - Propylites, 199 - Pseudo-dykes, 119 - Pumice, how formed, 68 - -- cause of white colour of, 71 - -- floating on ocean, 73 - -- on ocean-beds, 73 - Pumice-cones, 154 - _Puy de Pariou, Auvergne_, 193, 194, figs. 82, 83 - Puzzolana, 89 - - RAIN, accompanying volcanic outbursts, 30 - Rate of movement of lava-streams, 97 - Rath, Professor Vom, 72 - Red clay of ocean-beds, 74 - Red Mountains, Skye, 144 - Reservoirs beneath volcanoes, 145 - Reyer, Dr. Ed., experiments of, 125, 160 - Reykjanes, eruption of, in 1783, 102 - Rhyolites, 50, 59 - Richthofen, Von, cited, 196, 199, 200, 205 - _Rocca-Monfina_, 178, fig. 77 - -- --, 204 - Rock-masses, movements of, 288 - Rocky Mountains, 103 - -- -- volcanoes of, 201 - Rotomahana, sinter-terraces of, 185 - _Ropy-lavas_, 98, fig. 24 - - _SALINA, section in_, 132, fig. 53 - Sandwich Islands, lavas of, 125 - San Sebastiano, destruction of, 26 - _San Stephano, section in_, 131, fig. 50 - Santorin, 42 - _Sarcoui, Grand Puy of_, 126, fig. 44 - Sciarra del fuoco, 13 - Scoria, how formed, 68, 70 - Scoria-cones, altered by acid gas, 155 - -- breached, 156 - -- characters of, 153 - -- preservation of, 155 - -- red colour of, 154 - _Scoria-cone in Vesuvius_, 122, fig. 39 - _Scoria-cone near Auckland, N. Z._, 1656, fig. 66 - Schmidt, referred to, 153 - Schreibersite, 314 - Scrope, Mr. Poulett, cited, 5, 69, 106, 135, 198, 205, 212, 238, 289 - Sea of Azof, mud-volcanoes of, 182 - Secchi, Father, cited, 362 - Shiant Isles, 105 - Silica, presence in lavas, 47 - Silicates in lavas, 47 - Silicon, proportion in lavas, 47 - Siliceous sinter, deposits of, 220 - Silver, spitting of, 355 - Silvestri, Professor, cited, 230 - Similarity of lavas of different ages, 260 - _Sinter-cones, forms of_, 183, fig. 79 - Skye, dissected volcano of, 144 - Slags, compared with lavas, 46 - Smith, Lawrence, cited, 320 - Smoke, appearance of, due to steam, 2 - Snowdon, 274 - _Solar prominences_, 364-366, figs. 92, 93, 94 - Solfatara of Naples, 214 - Solfatara-stage of volcanoes, 215 - Somma, 133 - -- crater-ring of, 83 - Sorby, Mr. H. C., referred to, 59, 252 - Spallanzani, early researches of, 4 - -- observations on Stromboli, 8 - -- cited, 39, 367 - Specific gravities of lavas, 49 - -- -- of glassy and crystalline rocks, 59 - Spectroscope in vulcanology, 41 - Spectrum-analysis, results of, 311 - Specular-iron, deposited on lava-streams, 110 - Sperenberg, boring of, 341 - _Sphærulites_, 54, _Frontispiece_ - Sporadosiderites, 316 - Stability of crust of globe, 326 - Staffa, Isle of, 106 - Steenstrup, cited, 319 - Steam-engine compared to volcano, 8 - Steam, emitted by lava of Vesuvius, 27 - Sternberg, referred to, 113 - St. Kilda, 181 - Stokes, Professor, 65 - St. Paul, Island of, 180 - Stromboli, 42, 158 - -- apertures at bottom of crater, 15 - -- appearances in crater of, 16 - -- -- at night, 10 - -- compared with Vesuvius, 23 - -- crater of, 13 - -- dependence of eruptions on atmospheric conditions, 34 - -- eruption of, 14, fig. 4 - -- general features of, 11 - -- map of, 11, fig. 2 - -- observations by Spallanzani, 8 - -- resemblance to flashing light, 10 - -- _section of_, 13, fig. 8 - -- soundings around, 12 - -- vapour-cloud above, 9 - -- violent eruptions of, 23 - Strombolian stage, 23 - Stufas, nature of, 217 - Submarine volcanoes, 179 - Subterranean forces, beneficial effects of, 303 - Subsidence in centre of volcanoes, 165 - Sulphur, absorption of water by molten, 356 - -- deposited on lava-streams, 110 - -- how formed at volcanoes, 18 - -- not the cause of volcanic outbursts, 18 - _Surfaces of lava-streams_, 97-99, figs. 22, 23 - _Sun-spots_, 361-363, figs. 89, 90, 91 - Syenite, 59 - Syssiderites, 316 - Szabo, Professor, cited, 199 - - TACHYLYTE, 59 - Tertiary volcanoes of British Islands, 276 - _Terraces, sinter- and travertine-formation of_, 185, fig. 80 - Temperature, increase in deeper parts of earth's crust, 335 - -- rate of increase in different areas, 340 - Teneriffe, 44, 151 - _Teneriffe_, 178, fig. 77 - -- _peak of_, 175, fig. 73 - Tenon-and-mortise structure in basaltic columns, 107 - Theodosius and Vulcano, 3 - Thunder, accompanying volcanic outbursts, 28 - Trachytes, 49, 50, 69 - Trap-rocks, origin of, 241 - Trass, 90 - Travertine or Tibur-stone, 184 - -- deposits of, 220 - Triassic volcanoes of British Islands, 275 - Tridymite deposited on lava-streams, 110 - Troilite, 314 - Troost, cited, 355 - Tufa, or tuff, 90 - Tuff-cones, character of, 157 - -- denudation of, 157, 158 - Typhon, fable of, 3 - - ULTRA-BASIC lavas, 50, 66 - -- rocks, 317 - - VAL DEL BOVE, Etna, 133, 180, 209 - -- -- _dykes in_, 134, fig. 55 - Vapour-cloud over Vesuvius, 26, 29 - -- -- Stromboli, 9 - Ventotienne, Island of, section at, 130, fig. 49 - Vesuvius, 37 - -- changes in form of, 81 - -- compared with Stromboli, 23 - -- _crater of in 1756_, 84, fig. 14 - -- -- _of in 1767_, 85, fig. 16 - -- -- _of in 1822_, 82, fig. 13 - -- -- _of in 1843_, 86, fig. 16 - -- detonations at, 26 - -- early history of, 83 - -- eruption of year 79, 84 - -- -- of 1822, 69 - -- -- of April 1872, 24 - -- -- of October 1822, 24 - -- ejected blocks of, 45 - -- first eruption of, 7 - -- form of, 166 - -- fossils of, 45 - -- growth of cone of, 80 - -- history of, 204 - -- last eruption of, 7 - -- lava-stream of 1855, 101 - -- lava-streams of 1858, 1872, 97 - -- lavas of, 104 - -- minerals of, 46 - -- -- ejected at, 149 - -- observatory on, 24, 37 - -- outlines of, 87 - -- pine-tree appendage of, 29 - -- scoria-cones in lava, 122 - -- -- on lava of 1855, 153 - -- steam emitted by lava of, 27 - -- vapour-cloud over, 26, 29 - Vesuvian stage, 23 - -- _eruption, photograph of_, 24, fig. 5 - Viscid lavas of Lipari Islands, 94-96 - Vitreous lavas, devitrification of, 259 - Volcanic action, laws of, 32 - -- bombs, 70, 71 - -- cycles, nature of, 221, 222 - -- -- duration of, 223 - -- cones, internal structure of, 115-122 - -- -- _experimental illustration of formation of_, 120, fig. 37 - -- -- limits to height of, 166 - -- -- form of, 152 - -- -- dimensions of, 152 - -- -- irregular development of, 90 - -- -- slopes of sides of, 91 - -- -- composed of ejected rock-fragments, 156 - -- -- curved slopes of, 156 - -- _débris_ on sea-bottom, 240 - -- dust, fineness of, 69 - -- districts, areas of upheaval, 245 - -- ejections, alteration of, 258 - -- eruptions, compared to ebullition, 19, 20 - -- forces, compensate for denudation, 283 - -- -- intensity at former periods, 278 - -- -- necessity for action of, 285 - -- -- shifting of from one area to another, 277 - -- mountains, origin of conical forms, 89 - -- -- mode of growth, 89 - -- phenomena of the past similar to those at present, 273 - -- products, order of appearance of, 198, 199 - Volcanic rocks, 61 - -- -- similarity of ancient and modern, 253 - Volcano, origin of name, 3 - -- craters of, 167 - -- Island of. _See_ Vulcano. - -- compared to steam-engine, 8 - Volcanoes, blocks, ejected from, 45 - -- built up of ejected fragments, 74 - -- destruction caused by, 281 - -- dissected by denudation, 115, 139 - -- erroneous ideas concerning, 1 - -- ejection of different materials from, 205 - -- known to ancients, 3 - -- life-history of, 186 - -- number of, 224, 225 - -- of Africa, 227 - -- of America, 236 - -- of Asia, 236 - -- of Bohemia, 126 - -- of Central Asia, 236 - -- of Central Pacific, 236 - -- of Europe, 227 - -- of Hungary, 126 - -- position in relation to mountain chains, 243 - -- popular ideas concerning, 1 - -- reservoirs beneath, 145 - Volvic lava of, 103 - Vose, cited, 346 - Vulcan, forge of, 3 - Vulcano, island of, 3, 158 - Vulcano and Theodosius, 3 - _Vulcano_, 178, fig. 77 - -- _and Vulcanello, view of_, 43, fig. 6 - -- chemical deposits at, 44 - -- eruption in 1786, 43 - -- -- in 1873, 43 - -- -- _lava-stream in_, 95, fig. 19 - -- --, 103, fig. 27 - -- _plan of_, 195, fig. 85 - -- _section of volcanic cone in_, 116, fig. 35 - -- section in, 129 - -- shifting of centre of eruption in, 196 - _Vulcanello, craters of_, 197, fig. 86 - Vulcanology, origin of the science, 4 - -- earliest treatise on, 5 - - Walferdin, M., cited, 340 - Water in lavas, 353 - -- penetration through rocks, 358 - -- presence of in lavas, 102 - -- and saline solutions in cavities of crystals, 63 - Werner, cited, 201 - Western Isles of Scotland, 103, 139, 142 - -- -- volcanoes of, 212 - Whymper, Mr., 69 - Woodward, Mr., experiments of, 119 - Wrekin, ancient volcanic rocks of, 259 - - YOUNG, Professor, cited, 365 - - ZEOLITES, formation of, 150 - - - - PRINTED BY - - SPOTTISWOODE AND CO. 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